Settle an order via bank transfer by uploading a receipt image.
Multipart upload. The payment_key value matches the key of a
bank-transfer-type entry in the order's online_payments[] array —
read it off GET /orders/{id} first. Receipts must be JPEG or
PNG, max 5 MB.

Like POST /orders/{id}/payment, this requires the order to have
an existing bill (i.e. the merchant must have accepted it).