Make Quick Payment an independent action button like ‘Take Payment’ that allows the user to enter a Payment Method (the same that are available when editing Invoice details) and a Ref Number/Check Number/Appr Code. These fields can remain optional, but would greatly increase support for external (non-RepairShopr) points of sale by allowing better documentation.

As it is now, a Ref Number/Check Number can be added to an Invoice after a Quick Payment is applied by going to Change Invoice Details. Even the Payment Method can be changed from Quick to Other, which is what we are using now to indicate that we have processed a payment using our CC machine.
Some simple tweaks could make this process much easier and clearer.