Marks an open (or applicable) invoice as uncollectible. The invoice must belong to the merchant account resolved from the secret key. Response data matches the invoice-by-id shape (including optional expand for nested objects). On success, status is uncollectible, markedUncollectibleOn is set, autoAdvance is false, and nextPaymentAttempt is null as enforced by the payment service. Line-item price, accountDiscounts, and accountFees follow the same expand pattern as GET invoice.