Finalizes a draft invoice by ID. The invoice must be valid, not soft-deleted, and belong to the merchant account resolved from the secret key. Request body — autoAdvance: Optional. If omitted, treated as true. This controls the immediate finalize path only (when autoFinalizeAt is omitted): when true, GETTRX may open the invoice and schedule automatic collection according to the invoice collectionMethod. When false, the invoice is opened and numbered without scheduling automatic collection or recurring-engine follow-up (currentRecurringEngineProcess cleared). Request body — autoFinalizeAt: Optional. Unix timestamp (seconds) for when the recurring engine should run the finalize step. Cannot be used with autoAdvance = false. When autoFinalizeAt is provided, the platform does not apply the immediate autoAdvance = true finalize rules below (no open transition, no invoice number allocation, no nextPaymentAttempt / autoSendInvoiceAt / PAY_INVOICE / SEND_INVOICE from this call). It updates only these fields on the invoice (the invoice remains draft until the engine finalizes it): - autoAdvance = true - autoFinalizeAt = the Unix timestamp sent in the request - currentRecurringEngineProcess = FINALIZE_INVOICE - lastUpdated = current server time (NOW()) When autoFinalizeAt is omitted — autoAdvance = true (default): - collectionMethod = charge_automatically: autoAdvance = true, status = open, finalizedOn = now, autoFinalizeAt = null, invoiceNumber generated, nextPaymentAttempt set per platform automatic-collection policy, autoSendInvoiceAt = null, currentRecurringEngineProcess = PAY_INVOICE. - collectionMethod = send_invoice: autoAdvance = true, status = open, finalizedOn = now, autoFinalizeAt = null, nextPaymentAttempt = null, autoSendInvoiceAt set per platform invoice-send policy, invoiceNumber generated, currentRecurringEngineProcess = SEND_INVOICE. autoAdvance = false (only when autoFinalizeAt is omitted): autoAdvance = false, status = open, finalizedOn = now, autoFinalizeAt = null, nextPaymentAttempt = null, autoSendInvoiceAt = null, invoiceNumber generated, currentRecurringEngineProcess = null. Response data matches the invoice-by-id shape. Line-item price, accountDiscounts, and accountFees follow the same expand pattern as GET invoice when expand is supported.