Buy credits for the account owning the API key, paid by the caller itself over the Machine Payments Protocol (MPP, https://mpp.dev). No checkout page and no card form.

The first call answers 402 Payment Required with a WWW-Authenticate header carrying two Payment challenges, one for USDC on Tempo (method "tempo") and one for card or Link via a Stripe shared payment token (method "stripe"). The client pays one of them and repeats the same request with the payment credential; any MPP client (mppx, the Stripe Link CLI) does this automatically. The repeated request answers 200 and the credits are on the balance at once.

Rules: $0.02 per credit, minimum 25 credits, maximum 50,000 per call. Card top-ups are capped at $100 per account per rolling 24 hours; over the cap the 402 offers USDC only, which is uncapped. A trial account moves to the Starter pay-as-you-go price table on its first top-up and no subscription is created. A payment credential never credits twice: a replay answers 200 with status "already_credited".

Pay from the command line with a USDC wallet (npx -y mppx@0.9.2 https://api.myotp.app/v1/topup -X POST -H "x-api-key: KEY" -H "content-type: application/json" -d '{"credits":100}') or with a Link agent wallet (npx -y @stripe/link-cli mpp pay https://api.myotp.app/v1/topup -X POST -d '{"credits":100}' -H "x-api-key: KEY" --context "Buying MyOTP.App credits to send one-time passcodes over SMS, WhatsApp and Telegram for phone verification in my app.").