We need a way to beable to access a "processing fee" only a registration but only if theyre registration costs money and not be charged if a no charge registeration (aka no extras picked and couple etc)
We did it before with an extra field that was mandatory with the charge but itll charge everyone the fee not just people that actually process money.
You can't setup a custom field to be variable on anything else except for user selections. You'd have to think this through and setup your existing fee custom field to be CONDITIONAL on something else... example...
If the user selects "YES" on Field A, then that means they will get the extra fee and their registration is NOT free. So if that was the selection, then the processing fee field will be displayed... and required (needs set as required in the default options for that field).
Your situation may or may not be that simple, but that is the route you need to go. Surely you can figure out some method where you can make that work for you. That's your only option though.