Check whether that is permitted in your area.  Some jurisdictions make such fees illegal.

Technically, a script can be used to add such an amount as a miscellaneous charge.

Do you have an example of sure a script?

Noted on the legal issues. Thanks!

*Such not sure

There are scripting examples in the BOI forum, and learning resources in a few places, if that interests you.

The important question is, how would the program know to automatically add a credit card processing charge and how would the program know the order writer is finished with the order.