SFS sample checkout (test tool)

Builds and submits a real order to the live production API using the token from the OAuth flow. Products are fetched live from /listskus and shipping methods from /listshippingmethods. Countries and document types are currently a small static placeholder list (see note below) because /listcountries and /listdocumenttypes have been persistently rate-limited (HTTP 429) during testing - this is a deliberate, temporary deviation from the guide's s.4 "don't hard-code reference data" guidance, not an oversight.

Countries and document types are hardcoded right now, not fetched. /listcountries and /listdocumenttypes have been returning 429 (Too Many Requests) consistently for over 30 minutes while /listskus succeeds fine, so calls to those two endpoints have been switched off to stop making the situation worse. The list below is only the single example value from the Dev Guide s.4.2/4.4 (United Kingdom / GB, and document type ID 69 "Example document type") - it is almost certainly missing most of the real countries and document types your account actually has, and may not be safe to submit real orders against beyond quick smoke-testing. Swap in the real lists (paste a captured /listcountries and /listdocumenttypes response, or re-enable the live fetch in the code) once the rate limit is confirmed clear or explained.