You can create ant expense account to handle the credit card merchant fees or in this case an income account to record the additional revenue you are taking in for the processing fees. Payments received by credit card should not be handled as Cash because they are not cash. There is a process in Sage 50 to handle credit cards. go to Credit Card settings and set up the credit cards you will accept. 

Just be cautious of one other thing that could be a major concern. Charging a fee for using a credit card could violate your merchant agreement. The processing companies are very concerned with this type of violation and yes they have pulled/nullified their contracts with vendors.

You should consider using a price increase to cover your costs and then offer a discount for cash. Where do cheques fall in this process as they cost significant dollars as well depending on your bank and the banking methods you use.