Invoicing: Adding a write off category under invoices

Invoicing: Adding a write off category under invoices

Does anyone have a way to add type of 'write off' under the invoice options?
I would like to be able to keep track (in this system,) of how much patients are using their cards and how much I am paying in fees specifically. Anyone aware of a way to add 'Merchant fee' to the drop down list of 'contractual, bad debt, small amount and other'?

Many thanks!