Credit memo and invoice totals are derived dynamically from the order fees, not statically from a table like `custom_order_fees`

[damienwebdev]

Describe the bug
With the new multi-invoice and multi-creditmemo features in v1.2.1, I was trying to determine how much fee was billed on a given invoice/creditmemo programatically. This led me to the order totals block in the admin where credit memo and invoice fees are computed dynamically. This makes it very difficult to determine the value invoiced/refunded on a frontend UI.

To Reproduce
Steps to reproduce the behavior:

1. Create an order with a custom fee
2. Invoice a part of the order
3. Credit the invoice from step 2.
4. Struggle to determine how to compute the cost returned on a frontend UI.

Expected behavior
I would like either:

1. Static values stored for creditmemos and invoices so that fee computation is a simple read
2. A isolated class for fee computation for a given Invoice object or CreditMemo object.

[JosephLeedy]

Hello @damienwebdev! Thank you for raising this issue. I'll look into this further as time permits and see which option would work the best.

For clarity, this is the formula used to calculate the fee amount for invoices and credit memos:

(invoice_total ÷ order_total) × fee_amount