Initiate a full payment for an approved Opened Payable. The paying Account's own saved ExternalBankAccount or verified PaymentCard is debited; method: Ach describes the Payee-side disbursement leg, not a restriction on the payer-side funding source. Payer-subject cards and otherwise unresolvable sources return 404, changing the bound source on a retry returns 409, and shouldEnableAutoPay: true returns 422. Payee-scoped payout settings route funds to one or more Payee-subject external bank accounts. Works whether or not the Payee has linked a Wingspan Account. Payment initiation durably reserves the amount, funding source, and routes. A Returned Payable is terminal and replacement payment requires a new Payable; a non-terminal reservation requires Wingspan operator review. For a linked Payee, collaborator splits use the established collaborator-aware V1/V2 AP engine. Direct V3 routing for an unlinked Payee carrying collaborator splits is not yet supported and returns 422. Returns 409 EligibilityBlocked with detailCode payments.EngagementNotPaymentsEligible when the related engagement has incomplete payment eligibility requirements.

Requires a session with recent MFA step-up for HighRiskWriteAction. If step-up is missing or expired, this operation returns 403 StepUpMfaRequired; create and verify an MFA challenge at /v3/platform/mfa-challenges with requiredFor: "HighRiskWriteAction", then retry.