Adds one payment to a metered order, to charge the customer for usage. The payment joins the order's schedule as a new scheduled payment.

A scheduled payment is a planned future collection on an instalment, subscription or metered order. It shows as one billing event in the order's payment timeline. A metered order has no schedule of its own: each payment you add here is one scheduled payment.

1. Send the amount to charge. Choose when to collect it with process_date.
2. You get back the new scheduled payment, with status scheduled. Nothing is charged in this request.
3. BillPro collects it on its date. With now, the next collection run charges it. To charge it straight away and see the result, execute it with the returned id.

• The amount is saved as you send it. BillPro doesn't work it out from a product.
• The currency is always the order's currency. There's no currency field: one sent is ignored.
• Your note is saved in the order's notes. It isn't stored on the payment.
• Other fields, such as product_id and qty, are ignored.
• Only metered orders accept this request. Other order types return 422.

To cancel the payment before it's collected, skip it.