Retrieve a payment fee summary

Finishes an `initiated` payment once the shopper has done their step: approved at PayPal or Amazon Pay, passed a 3-D Secure check, or confirmed in Stripe.js. The amounts are worked out again from the cart or invoice first, so a changed shipping or tax is charged. On success the cart or invoice closes, the order is made, events fire and the receipt goes out. If a PayPal total went up too much since approval, the payment stays `initiated` with a new `response_data.redirect_url` for the buyer to approve again. Two commits at once (the shopper's browser and the provider's webhook) run one after the other, and the second gets the first one's result. A payment that is already `pending` comes back as it is. A `completed` or `cancelled` payment is `422`. The Comecero-hosted 3-D Secure page commits the payment itself, so get the payment first and commit only while it is `initiated`. Any token type may call it, a limited token too, for any payment of the account: a shopper may come back in another browser. A limited token that is not bound to the payment gets only `object`, `payment_id`, `success`, `status`, `payment_method.type` and `response_data.redirect_url`. A token that may not create or update payments gets `401`. The body is optional.