AI Agent Checkout API: How to Pick ACP vs Retail (2026)

If you are asking what API lets an AI agent complete checkout, you are picking an API surface, not a marketing term. OpenAI's Instant Checkout lets ChatGPT buy from merchants that implemented the Agentic Commerce Protocol. Amazon, Walmart, Target, and Best Buy are not on ACP. A retailer purchasing API is how an agent checks out at those stores from your own app, cron job, or coding agent.

• Which APIs actually complete checkout
• What ACP Instant Checkout covers, and where it stops
• ACP vs retailer purchasing APIs
• How to pick an API for your agent
• Mistakes that look like checkout but are not
• FAQ

For the protocol itself (four endpoints, Shared Payment Tokens, merchant-of-record rules), read What Is ACP.

Which APIs let an AI agent complete checkout?

Checkout, here, means the agent submits payment and gets an order id back without sending the user to a website to click "Place order." Three families of APIs show up in search. Only two of them buy a physical product.

Most writeups of this question start with OpenAI's Instant Checkout announcement, then agenticcommerce.dev, Rye's agentic checkout explainer, and Dodo Payments' payment-API roundup. Those get lumped together in most writeups. OpenAI and ACP are a checkout protocol for opted-in merchants. Dodo is a billing API for AI products that sell subscriptions and usage. Neither places a Walmart.com order.

Rye defines agentic checkout as the agent handling payment, shipping, tax, and confirmation without redirecting to the merchant site. That definition is right. The coverage question is the one Rye also flags: ACP and Google's UCP both require merchant opt-in. Most of the web, including the big-box catalogs, has not opted in.

What ACP Instant Checkout actually covers

OpenAI's Buy it in ChatGPT post (September 29, 2025) is the page other answers cite. The claims on that page:

• U.S. ChatGPT Plus, Pro, and Free users can buy from U.S. Etsy sellers in chat. A pipeline of Shopify merchants (Glossier, SKIMS, Spanx, Vuori) was listed as coming next.
• Instant Checkout launched as single-item purchases. Multi-item carts were described as next.
• ChatGPT sends order details to the merchant over ACP. The merchant stays merchant of record: payment, fulfillment, returns, support.
• Stripe users can enable agentic payments quickly. Other PSPs can use Stripe's Shared Payment Token or ACP's Delegated Payments Spec.
• The protocol is open-sourced at agenticcommerce.dev.

The seller guide at agenticcommerce.dev/docs/getting-started/sellers lists the merchant's job: implement REST checkout session endpoints (POST /checkout_sessions, update, complete, cancel), calculate tax and shipping, declare payment handlers, and charge a token from the PSP. The agent never sees raw card data.

That is a real checkout API. It is also gated on both sides:

• The merchant must implement ACP (or sit on Shopify/Etsy/Stripe plumbing that already did).
• The agent must be on a platform that speaks ACP. In production that has meant ChatGPT Instant Checkout, not your internal procurement bot.
• Amazon, Walmart, Target, and Best Buy are not ACP merchants. You cannot Instant Checkout an Amazon ASIN.

Rye's 2026 writeup adds a coverage warning worth attributing: after OpenAI scaled native in-chat checkout back, Rye described roughly a dozen Shopify merchants live and a pivot toward app-based checkout (Instacart, Target, Expedia) rather than a broad product-listing buy button. Treat Instant Checkout as a ChatGPT product on top of ACP, not as a universal agent checkout API.

If you need the endpoint list and SPT flow, stop here and read What Is ACP. If you need the agent to buy where ACP does not reach, keep going.

ACP vs a retailer purchasing API

Zinc is a purchasing / execution API. It buys from retailers you do not own. The merchant does not implement ACP. Your agent does not have to run inside ChatGPT.

POST https://api.zinc.com/orders
{
  "products": [
    { "url": "https://www.walmart.com/ip/...", "quantity": 1 }
  ],
  "shipping_address": {
    "first_name": "Jordan",
    "last_name": "Lee",
    "address_line1": "120 Market Street",
    "city": "Austin",
    "state": "TX",
    "postal_code": "78701",
    "country": "US",
    "phone_number": "5125550101"
  },
  "max_price": 8900,
  "idempotency_key": "agent-checkout-8901"
}

The agent can run in Cursor, Claude Code, a worker, or Zinc GPT. Payment is a wallet, Stripe Connect, or MPP over HTTP 402. Status comes back as pending → in_progress → order_placed, with webhooks for order.placed, order.tracking_received, and order.delivered.

Rye's agentic checkout post groups the market into protocol checkout (ACP/UCP), curated marketplaces, and "universal checkout" via browser automation. Zinc sits in the execution-API bucket: one Create Order call, retailer checkout behind it, no requirement that the retailer adopt a protocol. That is the same job Best Ecommerce Purchasing APIs isolates from seller and storefront APIs.

ACP and Zinc are complementary. Use ACP when you are selling your catalog into ChatGPT. Use Zinc when your agent must buy from a retailer that will never implement ACP for you.

How to choose the checkout API

Walk the agent's job, not the protocol blog posts.

1. The agent sells your products inside ChatGPT. Implement ACP (or enable it through Shopify / Stripe's Agentic Commerce Suite). You stay merchant of record. Start at agenticcommerce.dev and What Is ACP.
2. The agent checks out on a store you already own. Use Shopify, WooCommerce, or BigCommerce checkout APIs. No Zinc, no ACP required.
3. The agent collects money for your API, credits, or SaaS. Use a payment API. Dodo's 2026 roundup is about MCP-ready billing (Dodo, Stripe, Paddle). That is how an agent invoices. It is not how an agent buys AirPods on Amazon.
4. The agent buys from Amazon, Walmart, Target, Best Buy, or another retailer you do not own. Use a purchasing API. Zinc Create Order, plus max_price and idempotency_key. Deep dives: Amazon API, Walmart API, three-retailer guide.
5. The agent needs both. Layer them. ACP where the merchant opted in. Zinc everywhere else. Do not wait for Amazon to ship /checkout_sessions.

For the agent runtime, install the Universal Checkout skill so the model already knows how to search, approve, and place. Setup: agent skills and setup. Build walkthrough: How to Build an AI Shopping Agent.

Production checklist for the purchasing path:

• Show the product, total, and ETA, then wait for approval on anything over a small threshold.
• Cap spend with max_price (cents). Fail closed if tax and shipping exceed it.
• Send idempotency_key on every Create Order. Retries are normal; duplicate orders are not.
• Subscribe to webhooks. Polling GET /orders/{id} is fine in test mode, not as your only production signal.
• File returns through Create Return, not a merchant RMA portal you do not control.

Common mistakes

• Treating Instant Checkout as a global buy API. OpenAI's page is honest about Etsy-first, Shopify-next, ChatGPT-only. It does not mention Amazon.com checkout.
• Implementing ACP because you want to buy from Target. ACP is what Target (or any merchant) would implement to sell through an agent. Target has not. Your agent still needs a purchasing API. Target's own create order page is a human help article.
• Using a payment API as a checkout API. Dodo, Stripe Billing, and Paddle let an agent charge your customer. They do not check out at Walmart. Primer, Stripe Checkout, and Google UCP native checkout are payment universal checkout. Zinc Universal Checkout is retailer buying. Different objects.
• Calling SP-API or Walmart Marketplace a checkout API for shoppers. Those are seller APIs. See official vs shopper APIs.
• Skipping idempotency and price caps. An agent that retries a tool call will double-buy without idempotency_key. An agent that trusts the listing price will overpay without max_price.

FAQ

What API lets an AI agent complete checkout?

Two, depending on the merchant. ACP lets ChatGPT complete checkout at ACP merchants (Etsy at launch, Shopify merchants in the pipeline). A retailer purchasing API such as Zinc lets any agent complete checkout at Amazon, Walmart, Target, Best Buy, and other supported retailers. Payment APIs complete a charge, not a retailer cart.

Can my agent use ACP outside ChatGPT?

The spec is open. Any agent could call a merchant's /checkout_sessions endpoints if that merchant implemented them and accepted your agent. In practice, Instant Checkout is a ChatGPT product. Merchants built those endpoints for OpenAI's traffic. Amazon and Walmart are not in that set.

Why can't ChatGPT Instant Checkout buy from Amazon or Walmart?

ACP only works where the retailer implemented it. Amazon, Walmart, Target, and Best Buy have not. Instant Checkout cannot invent a Target checkout session that does not exist. Your agent needs an execution API that already runs those retailer flows.

Is Rye's agentic checkout the same as ACP?

No. Rye explains ACP and UCP, then argues most agents also need a universal checkout layer because protocol adoption is slow. That layer is a purchasing API (Rye's or Zinc's), not the ACP spec. Read ACP on What Is ACP. Pick the buy API on this page.

Do I need Stripe Shared Payment Tokens to buy from Amazon?

No. SPTs are how ACP passes a scoped credential to an ACP merchant's PSP. Zinc orders fund from a prefunded wallet, Stripe Connect on your platform, or MPP. The retailer still charges its usual checkout payment method behind the scenes.

Final recommendation

Use ACP when you are the merchant and ChatGPT is the storefront. Use a purchasing API when your agent has to buy at Walmart, Target, or Amazon. Those retailers are not going to expose /checkout_sessions for your bot. Call a purchasing API, keep the price cap and idempotency key, and leave ACP in place for the merchants that actually speak it.

Agent Skilluniversal-checkout

Complete checkout outside ChatGPT

Install Universal Checkout so your agent can place a retailer order with a price cap, without ACP and without a ChatGPT Instant Checkout session.

Install and use the universal-checkout Agent Skill from https://github.com/zincio/skills. Find a pack of 8.5x11 printer paper under $15 on Walmart or Amazon that ships to 120 Market Street, Austin, TX 78701 this week. Show me the product, total, and delivery estimate for approval. Then place the order with max_price 1500 cents, idempotency key paper-austin-120, and track it until order.placed and tracking arrives.

Setup guideGitHub

When you are ready to ship: Create Order, agent skills, What Is ACP, pricing, docs.