Indeed, although giving a discount for paying with cash is often allowed. (Customers generally hate hidden fees more than they hate missing out on a discount.)
It's not a way to cheat the system though, since you still have to advertise the higher price. Even if you also say "5% discount for cash transactions".
it's funny, I mean, many of my suppliers of bandwidth and rackspace accept credit cards. No fee, usually.
I'm amused at this thread because I woke up this evening to a bill from a provider that I recently did an especially good job negotiating with, and the guy wants a 3% payment processor fee on top of the negotiated amount.
(I'm going to leave an envelope with a cheque at the co-lo for him; gotta go anyhow.)
Nearly every processor agreement stipulates that this is a no-no.