Define the billable unit
A request, record, token, second or successful response creates different incentives. Choosing the unit is the monetization decision — see API monetization with x402.
Agent Commerce & Security · PDF FIELD GUIDE · 2026
Pay-Per-Call API Guide
The Pay-Per-Call API Guide is a ScriptMasterLabs field guide for designing a purchase flow where price, unit, fulfillment, and refund rules are clear before payment. Its method: define the billable unit, quote before execution, make retries idempotent, and issue receipts — then run a six-step review covering limits, pricing, idempotency keys, payment-acceptance separation, timeout and refund rules, and usage records.
Pricing units, metering, receipts and customer-safe failure handling
SML-X402-0034-PAGE PDF$29 ONE TIMEINSTANT DELIVERY
THE OUTCOME
How do you design an API purchase flow with clear terms before payment?
The guide’s outcome is an API purchase flow where price, unit, fulfillment and refund rules are clear before payment.
Use this guide to: Review pay-per-call API pricing, request identity, delivery verification and failure handling.
CORE MODEL
What four ideas come before action?
The four ideas are defining the billable unit, quoting before execution, making retries idempotent, and issuing useful receipts.
Quote before execution
Return enough terms for the buyer to evaluate price, scope and expiry. For MCP servers specifically, see the MCP payments guide.
Make retries idempotent
Network failures should not silently create duplicate charges or deliveries.
Issue useful receipts
Correlate quote, payment, request, response and acceptance status.
ACTION CHECKLIST
What is the six-step review process?
The six steps are naming the unit and included limits, publishing deterministic pricing, using idempotency keys, separating payment from acceptance, documenting timeout and refund rules, and exposing usage and receipt records.
- Name the unit and included limits
- Publish deterministic pricing
- Use idempotency keys
- Separate payment from acceptance
- Document timeout and refund rules
- Expose usage and receipt records
PRIMARY SOURCES
How do you verify the moving parts?
New free verification utility: x402 Quote Inspector checks a pasted PAYMENT-REQUIRED header in your browser. It can reject mismatched payees, unsupported networks and out-of-budget amounts; separate evidence is still needed for provider identity, settlement and delivery.
The PDF includes a source map and explicit research boundary. Product and regulatory details can change; current official material controls.
x402 protocol documentation →x402 specification repository →Model Context Protocol specification →OWASP API Security Top 10 →TRUTH BOUNDARY
This guide does not determine legal, tax, sanctions or money-transmission obligations.
AI tools assisted drafting and layout. SCRIPTMASTERLABS is responsible for editorial structure and source selection. No personalized investment, legal, tax, medical, regulatory, advertising or cybersecurity advice.
FAQ
What should you know before you buy?
The product is a four-page PDF field guide delivered immediately after successful Stripe Checkout, sold as a one-time purchase.
What format is this product?
A four-page PDF field guide delivered immediately after successful Stripe Checkout.
Is this a subscription?
No. It is a one-time purchase.
Does this guarantee a result?
No. This guide does not determine legal, tax, sanctions or money-transmission obligations.
Pay per call api: what is included?
4 pages in a fixed-layout PDF: Usage pricing, request identity, delivery checks and failure handling. This is a compact field guide with review checks and source notes.