Hi Brumby,
We do a couple of things with regard to claiming money for bad guests.
Our PMS alerts us to the fact that the card is "bad", out of date, wrong number, so at least while they are checking in we have that first alert.
Secondly if the guest gives me the impression that they are "sus", we now take a $300 bond. If they are not prepared to pay the bond, then we are not willing to take them on, for we feel they must have something to hide!! I give them a receipt which they must return in the morning in order to get their bond back, and our office doesn't open till 8am.
Now to actually answer your question....
The fee you charge should at least cover the cost of the transaction.I would like look at your bank statement and see how much you are charged per transaction.