Tax of Record API

Overview

Outpost Tax of Record provides a REST API that automates tax registration, calculation, and filings while you maintain your own checkout and payment service providers (Stripe, Adyen, etc.). The integration is server-to-server — your frontend never calls Outpost directly.

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Integration Flow

Frontend→Your Backend→Outpost API

Frontend initiates checkout and calls your backend
Your backend authenticates with Outpost using OAuth2 client_credentials
Backend calculates tax via Outpost API and displays it at checkout
After payment succeeds, backend stores the transaction with Outpost
Outpost handles all tax filing, compliance, and liability

Security & Operational Principles

Server-only secrets — OUTPOST_CLIENT_SECRET must never be exposed to browsers or logs
Token management — Cache access tokens with TTL, refresh on expiry or 401 responses
Idempotency — Prevent double confirm/cancel calls using merchantTransactionReference tracking
Timeouts & retries — 5s timeout per request, retry transient errors (network/5xx) with bounded backoff
Production gates — Store calls only after PSP authorization/capture succeeds

Environment Setup

Variable	Value	Notes
API_BASE_URL	https://api.outpostanywhere.com	Base URL for all API endpoints
CLIENT_ID	Your client ID	Non-sensitive, can be inlined
OUTPOST_CLIENT_SECRET	—	Store in environment variables or secret manager

Authentication

Outpost uses OAuth2 client_credentials flow for server-to-server authentication. Your backend requests an access token using your client ID and secret, then includes it as a Bearer token in all API calls.

Flow

Request access token using client credentials
Parse response and extract access_token with expires_in
Cache token server-side with TTL (recommend expires_in - 300s buffer)
Refresh token on expiry or 401 responses
Include Bearer token in all subsequent API requests

Cache access_token with expires_in - 300s buffer; retry once after refresh on 401.

Token Response

{
  "access_token": "<JWT>",
  "expires_in": 86400,
  "token_type": "Bearer"
}

Authentication

# Step 1: Obtain an access token
curl -X POST \
  "https://access.outpostanywhere.com/oauth2/token" \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d "grant_type=client_credentials" \
  -d "client_id=YOUR_CLIENT_ID" \
  -d "client_secret=$OUTPOST_CLIENT_SECRET" | jq

# Export the token (example)
# export OUTPOST_ACCESS_TOKEN="eyJhbGciOi..."

# Sample response:
# {
#   "access_token": "<JWT>",
#   "expires_in": 86400,
#   "token_type": "Bearer"
# }

Run from your server. Never expose secrets in the browser.

Create Tax Calculation

POST/api/tax/calculations

Creates a tax calculation for a given set of line items and customer information. The calculation determines applicable tax rates based on the customer's location and can be used to create a transaction after the successful payment.

Request Headers

Parameter	Type	Required	Description
Authorization	—	Yes	Bearer token for authentication
Content-Type	—	Yes	application/json

Request Body

{
  "currency": "EUR",
  "customer": {
    "merchantCustomerReference": "CUST-12345",
    "firstName": "John",
    "lastName": "Doe",
    "email": "john.doe@example.com",
    "billingAddress": {
      "line1": "Keizersgracht 123",
      "line2": "Apt 4B",
      "city": "Amsterdam",
      "postalCode": "1015 CJ",
      "country": "NL"
    }
  },
  "lineItems": [
    {
      "merchantLineItemReference": "ITEM-001",
      "productCode": "TEST-SKU-005",
      "description": "Premium Subscription - Annual",
      "unitPrice": 99.99,
      "quantity": 1,
      "discountAmount": 10.00
    }
  ],
  "evidence": {
    "billingCountry": "NL",
    "ipAddress": "185.23.108.42",
    "paymentMethodCountry": "NL"
  }
}

Request Body Parameters

Parameter	Type	Required	Description
currency	string	Yes	ISO 4217 currency code (e.g., EUR, USD, GBP)
customer	object	Yes	Customer information object
customer.merchantCustomerReference	string	No	Your internal customer identifier
customer.firstName	string	No	Customer's first name
customer.lastName	string	No	Customer's last name
customer.email	string	No	Customer's email address
customer.billingAddress	object	Yes	Customer's billing address
customer.billingAddress.line1	string	Yes	Primary address line
customer.billingAddress.line2	string	No	Secondary address line
customer.billingAddress.city	string	Yes	City name
customer.billingAddress.state	string	Conditional	State/province code (required for US and CA)
customer.billingAddress.postalCode	string	Yes	Postal/ZIP code
customer.billingAddress.country	string	Yes	ISO 3166-1 alpha-2 country code
lineItems	array	Yes	Array of line items (minimum 1)
lineItems[].merchantLineItemReference	string	Yes	Your internal line item identifier
lineItems[].productCode	string	Yes	Product/service code for tax classification
lineItems[].description	string	Yes	Human-readable item description
lineItems[].unitPrice	decimal	Yes	Price per unit (must be ≥ 0)
lineItems[].quantity	integer	No	Quantity (default: 1, must be > 0)
lineItems[].discountAmount	decimal	No	Discount amount for this line item
evidence	object	Conditional	Tax evidence (required for non-US billing addresses)
evidence.billingCountry	string	Conditional	ISO 3166-1 alpha-2 billing country code
evidence.ipAddress	string	Conditional	Customer's IP address
evidence.paymentMethodCountry	string	No	Payment method country code

Note: For non-US billing addresses, evidence.billingCountry must match customer.billingAddress.country.

Response 201 Created

{
  "taxCalculationId": "f47ac10b-58cc-4372-a567-0e02b2c3d479",
  "createdAt": "2026-01-05T12:00:00Z",
  "expiresAt": "2026-01-05T12:30:00Z",
  "currency": "EUR",
  "subTotalAmount": 99.99,
  "discountAmount": 10.00,
  "netAmount": 89.99,
  "taxAmount": 18.90,
  "totalAmount": 108.89,
  "customer": {
    "firstName": "John",
    "lastName": "Doe",
    "email": "john.doe@example.com",
    "billingAddress": {
      "line1": "Keizersgracht 123",
      "line2": "Apt 4B",
      "city": "Amsterdam",
      "postalCode": "1015 CJ",
      "country": "NL"
    }
  },
  "lineItems": [
    {
      "lineItemId": "a1b2c3d4-5678-90ab-cdef-1234567890ab",
      "merchantLineItemReference": "ITEM-001",
      "productCode": "TEST-SKU-005",
      "description": "Premium Subscription - Annual",
      "unitPrice": 99.99,
      "quantity": 1,
      "discountAmount": 10.00,
      "itemSubTotalAmount": 99.99,
      "itemNetAmount": 89.99,
      "itemTotalAmount": 108.89,
      "tax": {
        "rate": 21.00,
        "taxAmount": 18.90,
        "currency": "EUR"
      }
    }
  ],
  "evidence": {
    "billingCountry": "NL",
    "ipAddress": "185.23.108.42",
    "paymentMethodCountry": "NL"
  }
}

Response Fields

Parameter	Type	Required	Description
taxCalculationId	string	—	Unique identifier for the calculation
createdAt	string	—	ISO 8601 timestamp when calculation was created
expiresAt	string	—	ISO 8601 timestamp when calculation expires
currency	string	—	Currency code
subTotalAmount	decimal	—	Sum of line item amounts before discounts
discountAmount	decimal	—	Total discount amount applied
netAmount	decimal	—	Amount after discounts, before tax
taxAmount	decimal	—	Total tax amount
totalAmount	decimal	—	Final amount including tax
customer	object	—	Customer information
lineItems	array	—	Calculated line items with tax details
evidence	object	—	Tax evidence used for calculation

Error Responses

HTTP	Code	Description
400	invalid_argument	Missing or invalid field — check field requirements
400	invalid_state	Please activate Outpost Tax of Record to process transactions
409	region_not_activated	Merchant not activated for the customer's tax jurisdiction
409	region_not_supported	Customer location outside supported jurisdictions
409	integration_not_configured	Tax compliance integration is not configured
401	—	Invalid or missing Authorization token

Code Example

# Step 2: Calculate tax for the transaction
curl -X POST "https://api.outpostanywhere.com/api/tax/calculations" \
  -H "Authorization: Bearer $OUTPOST_ACCESS_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "currency": "EUR",
    "customer": {
      "merchantCustomerReference": "CUST-001",
      "firstName": "John",
      "lastName": "Doe",
      "email": "john.doe@example.com",
      "billingAddress": {
        "line1": "123 Main St",
        "city": "Berlin",
        "postalCode": "10115",
        "country": "DE"
      }
    },
    "merchantTransactionReference": "TXN-001",
    "lineItems": [
      {
        "merchantLineItemReference": "ITEM-001",
        "productCode": "SKU-123",
        "description": "Sample Product",
        "unitPrice": 19.99,
        "quantity": 1
      }
    ],
    "evidence": {
      "billingCountry": "DE",
      "ipAddress": "192.168.1.1",
      "paymentMethodCountry": "DE"
    }
  }' | jq

# Response: 201 Created with taxCalculationId
# Save the taxCalculationId for the Store step

Create Tax Transaction

POST/api/tax/transactions

Converts a valid tax calculation into a confirmed tax transaction. This endpoint should be called when an order is finalized/paid. Only call after payment authorization/capture succeeds.

Request Headers

Parameter	Type	Required	Description
Authorization	—	Yes	Bearer token for authentication
Content-Type	—	Yes	application/json

Request Body

{
  "taxCalculationId": "f47ac10b-58cc-4372-a567-0e02b2c3d479",
  "merchantTransactionReference": "ORDER-2026-001234",
  "payment": {
    "processor": "stripe",
    "paymentReference": "ch_3Q1234567890abcdef"
  }
}

Request Body Parameters

Parameter	Type	Required	Description
taxCalculationId	string	Yes	ID of the tax calculation generated by Outpost to convert
merchantTransactionReference	string	Yes	Your unique order/transaction reference
payment	object	No	Payment processor information
payment.processor	string	No	Payment processor name (e.g., "stripe", "adyen")
payment.paymentReference	string	No	Payment processor transaction ID

Response 200 OK

{
  "transactionId": "e83a9c47-2b5d-4f8a-9c12-3d4e5f6a7b8c",
  "refunded": false,
  "createdAt": "2026-01-05T12:05:00Z",
  "currency": "EUR",
  "subTotalAmount": 99.99,
  "discountAmount": 10.00,
  "netAmount": 89.99,
  "taxAmount": 18.90,
  "totalAmount": 108.89,
  "merchantTransactionReference": "ORDER-2026-001234",
  "transactionDate": "2026-01-05T12:05:00Z",
  "customer": {
    "firstName": "John",
    "lastName": "Doe",
    "email": "john.doe@example.com",
    "billingAddress": {
      "line1": "Keizersgracht 123",
      "line2": "Apt 4B",
      "city": "Amsterdam",
      "postalCode": "1015 CJ",
      "country": "NL"
    }
  },
  "payment": {
    "processor": "stripe",
    "paymentReference": "ch_3Q1234567890abcdef"
  },
  "lineItems": [
    {
      "lineItemId": "a1b2c3d4-5678-90ab-cdef-1234567890ab",
      "merchantLineItemReference": "ITEM-001",
      "productCode": "TEST-SKU-005",
      "description": "Premium Subscription - Annual",
      "unitPrice": 99.99,
      "quantity": 1,
      "discountAmount": 10.00,
      "itemSubTotalAmount": 99.99,
      "itemNetAmount": 89.99,
      "itemTotalAmount": 108.89,
      "tax": {
        "rate": 21.00,
        "taxAmount": 18.90,
        "currency": "EUR"
      }
    }
  ],
  "evidence": {
    "billingCountry": "NL",
    "ipAddress": "185.23.108.42",
    "paymentMethodCountry": "NL"
  }
}

Error Responses

HTTP	Code	Description
400	invalid_argument	Missing or invalid field — check field requirements
400	invalid_state	Tax calculation has expired
400	invalid_state	Tax calculation has already been used
400	invalid_state	Tax transactions API can only be used for Tax of Record product
404	not_found	Tax calculation not found

Code Example

# Step 3: Store the transaction using taxCalculationId from calculate response
curl -X POST "https://api.outpostanywhere.com/api/tax/transactions" \
  -H "Authorization: Bearer $OUTPOST_ACCESS_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "taxCalculationId": "'"$TAX_CALCULATION_ID"'",
    "merchantTransactionReference": "TXN-001",
    "payment": {
      "paymentReference": "PAY-001",
      "processor": "stripe"
    }
  }' | jq

# Response: 200 OK with TaxTransactionResponse

Get Tax Transaction

GET/api/tax/transactions/{transaction_id}

Retrieves the details of an existing tax transaction.

Path Parameters

Parameter	Type	Required	Description
transaction_id	string	Yes	ID of the transaction

Response 200 OK

Returns a TaxTransactionResponse object (same structure as Create Tax Transaction response).

Error Responses

HTTP	Code	Description
404	not_found	Transaction not found

Get Transaction Refunds

GET/api/tax/transactions/{transaction_id}/refunds

Retrieves all refunds associated with a transaction.

Path Parameters

Parameter	Type	Required	Description
transaction_id	string	Yes	ID of the original transaction

Response 200 OK

[
  {
    "refundTransactionId": "d1e2f3a4-5b6c-7d8e-9f0a-1b2c3d4e5f6a",
    "merchantRefundReference": "REFUND-2026-000456",
    "refundDate": "2026-01-10T15:30:00Z",
    "refundLineItems": [
      {
        "lineItemId": "a1b2c3d4-5678-90ab-cdef-1234567890ab",
        "refundLineItemReference": "REFUND-ITEM-001",
        "refundReason": "Customer requested cancellation",
        "refundItemSubTotalAmount": 99.99,
        "refundItemTotalAmount": 108.89,
        "refundTax": {
          "rate": 21.00,
          "taxAmount": 18.90,
          "currency": "EUR"
        }
      }
    ],
    "createdAt": "2026-01-10T15:30:00Z",
    "currency": "EUR",
    "subTotalAmount": 99.99,
    "taxAmount": 18.90,
    "totalAmount": 108.89
  }
]

Error Responses

HTTP	Code	Description
404	not_found	Transaction not found

Create Refund Transaction

POST/api/tax/transactions/{transaction_id}/refunds

Creates a refund for one or more line items in an existing transaction.

Path Parameters

Parameter	Type	Required	Description
transaction_id	string	Yes	ID of the original transaction

Request Body

{
  "merchantRefundReference": "REFUND-2026-000456",
  "refundDate": "2026-01-10T15:30:00Z",
  "refundLineItems": [
    {
      "lineItemId": "a1b2c3d4-5678-90ab-cdef-1234567890ab",
      "refundLineItemReference": "REFUND-ITEM-001",
      "refundReason": "Customer requested cancellation"
    }
  ]
}

Request Body Parameters

Parameter	Type	Required	Description
merchantRefundReference	string	Yes	Your unique refund reference
refundDate	string	No	ISO 8601 date/time of the refund
refundLineItems	array	Yes	Array of line items to refund (minimum 1)
refundLineItems[].lineItemId	string	Yes	ID of the original line item
refundLineItems[].refundLineItemReference	string	Yes	Your unique reference for this refund item
refundLineItems[].refundReason	string	Yes	Reason for the refund

Response 200 OK

{
  "refundTransactionId": "d1e2f3a4-5b6c-7d8e-9f0a-1b2c3d4e5f6a",
  "merchantRefundReference": "REFUND-2026-000456",
  "refundDate": "2026-01-10T15:30:00Z",
  "refundLineItems": [
    {
      "lineItemId": "a1b2c3d4-5678-90ab-cdef-1234567890ab",
      "refundLineItemReference": "REFUND-ITEM-001",
      "refundReason": "Customer requested cancellation",
      "refundItemSubTotalAmount": 99.99,
      "refundItemTotalAmount": 108.89,
      "refundTax": {
        "rate": 21.00,
        "taxAmount": 18.90,
        "currency": "EUR"
      }
    }
  ],
  "createdAt": "2026-01-10T15:30:00Z",
  "currency": "EUR",
  "subTotalAmount": 99.99,
  "taxAmount": 18.90,
  "totalAmount": 108.89
}

Error Responses

HTTP	Code	Description
404	not_found	Transaction or line items not found

Get Transaction Invoice

GET/api/tax/transactions/{transaction_id}/invoice

Retrieves the invoice file for a transaction.

Path Parameters

Parameter	Type	Required	Description
transaction_id	string	Yes	ID of the transaction

Response 200 OK

{
  "b2cInvoice": {
    "fileName": "invoice-e83a9c47-2b5d-4f8a-9c12-3d4e5f6a7b8c.pdf",
    "url": "https://storage.outpostnow.com/invoices/..."
  }
}

Response Fields

Parameter	Type	Required	Description
b2cInvoice	object	—	B2C invoice information
b2cInvoice.fileName	string	—	Name of the invoice file
b2cInvoice.url	string	—	URL to download the invoice

Response 202 Accepted — Invoice is still being generated. Retry after a short delay.

Integration Flow

The full checkout integration follows a linear sequence: authenticate, calculate tax, process payment, then store the transaction.

Sequence

Get/refresh access token (cache with TTL)
Calculate tax → capture taxCalculationId
Process payment with your PSP (Stripe, Adyen, etc.)
If payment succeeds → Call Store with taxCalculationId and payment details

Pseudocode

async function processCheckout(orderData) {
  // Step 1: Get auth token
  const token = await getAccessToken();

  // Step 2: Calculate tax
  const taxResponse = await calculateTax(orderData, token);
  const taxCalculationId = taxResponse.taxCalculationId;

  // Persist mapping for idempotency
  await persistCalculationMapping(orderData.orderId, taxCalculationId);

  // Step 3: Process payment with your PSP
  const paymentResult = await processPayment(orderData, taxResponse.taxTotal);

  // Step 4: Store transaction only if payment succeeds
  if (paymentResult.success) {
    await storeTransaction({
      taxCalculationId,
      merchantTransactionReference: orderData.orderId,
      payment: {
        paymentReference: paymentResult.paymentReference,
        processor: paymentResult.processor  // e.g., "stripe", "adyen"
      }
    }, token);
  }

  return { success: paymentResult.success, taxResponse };
}

Idempotency Controls

Store merchantTransactionReference → taxCalculationId mappings
Check existing mappings before creating new calculations
Implement mutex/locks for concurrent order processing
Use database constraints to prevent duplicate transactions
The merchantTransactionReference in Store prevents duplicate storage

Error Handling

Classify errors by HTTP status code and handle them accordingly. Never retry 4xx errors blindly — they indicate client-side issues that need to be fixed.

4xx — Client Errors

Do not retry blindly.

401 — Refresh token and retry once
400 — Log and fail fast (likely data validation issue)
404 — Calculation not found (check taxCalculationId when calling Store)

5xx — Server Errors

Temporary failures, safe to retry.

Safe to retry with exponential backoff
Max 3 retries with 1s, 2s, 4s delays
Include jitter to prevent thundering herd

Token Refresh on 401

// On 401 response:
// 1. Clear cached token
// 2. Request new access token
// 3. Retry the original request once with new token
// 4. If still 401, fail and alert

Structured Logging

{
  "timestamp": "2024-01-15T10:30:00Z",
  "level": "INFO",
  "message": "Tax calculated",
  "orderId": "order-12345",
  "taxCalculationId": "calc-67890",
  "currency": "USD",
  "taxAmount": 8.25,
  "requestId": "req-uuid-123",
  "duration": 1200
}

Metrics & Alerting

Success rates — Percentage of successful calculate/store operations
Latency — P95/P99 response times for each endpoint
Error rates — 4xx vs 5xx error breakdown by endpoint
Token refresh frequency — Monitor auth token lifecycle

Alert Conditions

Success rate drops below 95%
P95 latency exceeds 2 seconds
5xx error rate exceeds 1%
Store rate significantly lower than calculate rate
Token refresh failures

Testing & Rollout

Follow a phased rollout strategy to minimize risk. Start with calculate-only mode, then gradually enable store calls as you gain confidence.

Sandbox Testing

Authentication

Verify token acquisition and refresh

Calculate

Test with various customer/product scenarios

Store

Test successful payment flow with valid taxCalculationId

Error handling

Test timeout, retry, and 4xx/5xx scenarios

Rollout Strategy

Phase 1Calculate only (store disabled) to test tax calculation

Phase 2Enable Store for 1% of successful payments

Phase 3Gradually increase to 10%, 50%, 100%

RollbackFeature flags to disable Store calls quickly if needed

Acceptance Checklist

Auth token acquisition works

Token caching and refresh logic implemented

Calculate returns valid taxCalculationId

taxCalculationId properly stored with order

Store includes payment details (paymentReference, processor)

Store only called after successful payment

Timeout and retry logic implemented

Error handling covers 4xx and 5xx cases

Logging includes correlation IDs

Monitoring and alerting configured

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