ChamberMaster/MemberZone cannot apply a percentage charge to invoices to cover credit card fees due to the complexity of the implementation.
Surcharging, or passing the credit card fees of a transaction onto the purchaser requires adhering to regulations that can be quite complex. Calculating the surcharge must take into account multiple variables like state laws, shipping address, customer address, the payment 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 specific information on legality of passing on the credit card fees.