GrowthZone does not have the ability to apply a percentage charge to invoices to cover your credit card fees. We did investigate the option for a while, but due to the complexity of the implementation, it was decided that it was not a feature we would be able to develop:
Surcharging or the action of passing the credit card fees of a transaction onto the purchaser requires adhering to regulations that can be quite complex. The calculation of the surcharge has to take into consideration potential variables like state laws, shipping address, customer address, and the amount and the type of card used to avoid running afoul of the law and violating the merchant agreement.
Several states do not allow surcharging practices at all. Trying to simplify the calculated surcharge on a given transaction by using a flat percentage rate for all transactions paid by credit card is not possible because it is grossly non-compliant. At this time, it is not possible to logistically engineer and maintain a surcharge calculation tool accurate enough to remain in compliance.
Refer to the specific laws for your state for information on legality of passing on the credit card fees.