Shop Fast Amazon Execute

I want to execute an Amazon purchase through "Shop Fast Amazon Execute".

## Setup (skip if you already have Fast Marketplace set up)
1. Open the marketplace skill: https://staging.marketplace.fast.xyz/skill.md
2. Call amazon-quote to create a marketplace-saved quote.
3. Review the quote response and collect explicit buyer consent.
4. Call amazon-buy with the quoteId and buyerConsent using x402 payment for the quote amount.

## Available Endpoints

### Amazon Buy (Quote-bound testUSDC)
curl -X POST "https://staging.api.marketplace.fast.xyz/api/shop-fast-amazon/amazon-buy" \
  -H "Content-Type: application/json" \
  -d '{
  "quoteId": "quote_example",
  "buyerConsent": {
    "accepted": true,
    "acceptedAt": "2026-03-19T00:10:00.000Z"
  }
}'

Requires a quote created by amazon-quote. The marketplace records consent, order, and fulfillment state around the upstream purchase request.

### Amazon Quote (Free)
curl -X POST "https://staging.api.marketplace.fast.xyz/api/shop-fast-amazon/amazon-quote" \
  -H "Content-Type: application/json" \
  -d '{
  "quantity": 1,
  "productId": "B000EXAMPLE",
  "shipToCountry": "US",
  "paymentCurrency": "USDC"
}'

The returned quoteId is persisted by the marketplace and required by amazon-buy.

For paid endpoints: the first call returns 402. Authorize payment with your Fast wallet and retry with the payment signature header.

For free endpoints: call the marketplace route directly with the published method and request example. No payment headers are required.