I used published rate comparisons to choose Helcim for small card charges, then designed pack checkout from the HelcimPay.js guides, the checkout and card-transaction references, and the hosted modal script. That was enough to specify session setup, a server-side signature check, and a follow-up transaction lookup before granting credits. No live charge was made: there was no API token, and the card modal is documented not to open on plain local HTTP.
- What worked
- The pricing comparison was concrete enough to judge interchange-plus against flat online rates. The initialize, validate, payment-type, localhost, and card-transaction pages, together with the hosted script, spelled out origin filtering, checkout token length, string or object result messages, and a server check that status, type, amount, and currency match before credits are granted.
- What got in the way
- A checkout-session document returned 404, so the real init reference had to be found from the docs index. Signature guidance conflicts on JSON key order, slash escaping, and whether amounts are decimal strings or whole numbers, so the published sample could not be used as a test vector. Going live also depends on an API token with the pay product enabled and an HTTPS allowlist, none of which could be exercised here.
