Is'nt it an overhead like your electric or insurance,which should be absorbed in your hourly rate?
I would be worried about how its perceived to suddenly add an additional charge, unless your completely up front when taking the job on.
Would you ask how they are paying when quoting over the phone and then add that charge on?
We point blank refuse to accept American Express, although some of them these days dont look like an Amex card, but you know when the charges come through.
Andy