Create a payment method

Bacs Direct Debit payments are not instant and not final. The provider confirms the debit on or after the day the bank takes the money (`succeeded`: the money is collected, and the invoice is paid), then pays it out to the operator (`paid_out`); either can still be undone by `failed`, `late_failure_settled` or `chargeback_settled`. See ADR 0017 and ADR 0067. A Direct Debit against an invoice is requested for the day the customer's advance notice named, never earlier, or for the first day the bank can debit once that day can no longer be met; the day the provider then debits is `provider_charge_date`. When `invoice_id` is present, `currency` must equal the invoice's own currency and the invoice must be in a collectable status (`open`, `partially_paid`, `overdue`). For an offline recording (`rail=offline`) the `Idempotency-Key` header is REQUIRED — the recording settles synchronously, so a keyless duplicate has no later reconciliation point. A card or Direct Debit amount under what the instrument's provider collects in one payment is refused with `422` on `amount_minor`, issue `below_minimum` — 30p for a Stripe card and £1.00 for a Bacs Direct Debit in GBP (Stripe's and GoCardless's own minimums, per currency); both providers decline a smaller payment permanently. An offline recording has no minimum. An `invoice_id` must be an invoice of the customer the card or mandate belongs to: another customer's invoice is refused with `422` on `invoice_id`, before its status, currency or balance are judged. A `subscription_id` must be a plan of that customer's own (or the plan the named invoice itself carries); any other is `422` on `subscription_id`.