Save a payment method to a customer
Attach a tokenized card to a customer for later off-session (MIT)
charges. Only the card family produces a reusable token, so type is
restricted to CARD / APPLE_PAY / GOOGLE_PAY. May return REQUIRES_ACTION
with a 3DS challenge URL. No Payment is created — pure attachment.
Authorizations
Secret API key. sk_live_… (production) or sk_sandbox_… (sandbox).
Headers
Optional key (≤255 chars, e.g. a UUID v4) that makes this POST safe to retry: the first request executes and its response is cached for 24h; a retry with the SAME key replays that response instead of re-executing (no duplicate payment/subscription). A reused key with a different request → 422 IDEMPOTENCY_KEY_CONFLICT; a retry while the first is still in flight → 409. See the Idempotency guide.
255"5f3b9c2e-1a4d-4f2b-9c31-7e2a1b6d8c04"
Body
The customer to attach the instrument to. Required.
"cus_abc123"
Optional proactive-3DS browser fingerprint — authenticates the card up front so later off-session debits are SCA-exempt MITs. Omit to save without 3DS.

