API Documentation: The Checkout Endpoint – The "Million Dollar" Request

The /checkout endpoint isn't just code; it is the moment intent turns into revenue. Here is how to write documentation that prevents errors, handles payments safely, and ensures a seamless transaction every time. 

The Moment of Truth

In the architecture of any e-commerce platform, the checkout endpoint (often /v1/orders or /v1/checkout) is the "Money Endpoint."

It is the bottleneck through which every dollar of revenue must pass. Unlike a GET /products endpoint—where a failure is merely annoying—a failure at checkout is catastrophic. It means a lost sale, a frustrated customer, and potentially a damaged brand reputation.

For frontend developers and mobile engineers integrating your API, the checkout documentation is the manual for a bomb disposal unit. One wrong wire (parameter) and the whole thing blows up (declines).

This guide covers the essential components often missing from standard API documentation, ensuring your integrators can build a robust, high-converting checkout flow.

The "Pre-Flight" Context (Authentication & State)

Before a developer even looks at the JSON body, they need to know the rules of engagement.

State Dependencies

Checkout rarely happens in a vacuum. Your documentation must clarify:

• Is a Cart ID required? Can I hit this endpoint with a raw list of SKUs, or must I first create a cart via POST /carts?
• Token Scope: Does the API token require specific write:orders or payments scopes?
• Session Locking: Does hitting this endpoint "lock" the inventory temporarily?

Document This: “Note: This endpoint requires a valid cart_id generated within the last 30 minutes. The cart must not be empty.”

The Payload "Contract" (Validation & Dependencies)

Standard documentation lists fields. Great documentation explains relationships.

A checkout payload is complex. It contains customer PII, shipping addresses, and payment tokens. The biggest source of friction is Conditional Logic.

Handling Field Dependencies

You must explicitly document rules like:

• Billing Address: Optional unless the payment method is Credit Card (AVS check).
• Tax ID: Required only for B2B transactions or specific countries (e.g., Italy's Codice Fiscale or Brazil's CPF).
• Phone Number: Required for Express Shipping carriers (DHL/FedEx) but optional for Standard Mail.

Idempotency (The Safety Net)

This is the single most critical technical concept for payments, yet it is often missing from documentation.

The Scenario: A mobile user on a train clicks "Pay." The train goes into a tunnel. The request is sent, but the response is lost. The app automatically retries.

• Without Idempotency: The user is charged twice.
• With Idempotency: The server recognizes the "Retry Key" and returns the original success message without charging again.

How to Document It:

1. Define the Header: Explicitly state which header to use (e.g., Idempotency-Key or X-Request-ID).

2. Define the Behavior: Explain that sending the same key results in a replay of the previous response, not a new operation.

Asynchronous Processing (The "Pending" State)

In modern e-commerce, payment isn't always instant. Methods like Buy Now Pay Later (Klarna), SEPA Direct Debit, or Crypto take time.

Your documentation must prepare the developer for the "Pending" limbo.

• Synchronous Response: "Credit Card Approved" (200 OK).
• Asynchronous Response: "Payment Initiated" (202 Accepted). The order is created, but the status is pending_payment.

The Webhook Handoff: Your docs must link to the Webhooks section. Explain that for async methods, the final "Order Paid" confirmation will come via a webhook event (payment.success), not the immediate API response.

Granular Error Handling (The "Unhappy Path")

A generic 400 Bad Request or 500 Server Error during checkout is a nightmare for support teams. The client app needs to know exactly what to tell the user.

Your documentation needs a dedicated Error Codes Table specifically for checkout.