Accept underpayment

Underpayments can be programmatically accepted through this API endpoint in two different ways:

a. To accept an underpayment at current value, send only a checkout_id in the body request.

b. To get a quote for current value of an underpayment and then accept that quote:

1. First send checkout_id and auto_confirm: false, which will return a quote
2. To accept that quote, send checkout_id and quote_id (id from last step).