Hi. Have you thought about 3rd party payment portals like World Pay, etc like the previous post shows? A lot of retailers use them as its a way for customers to pay via Cards so that their details still get validated by the banking system but are not known to anyone, not even the retailer, all they will get is a confirmation of payment clearance to their account that the customer has paid.
I would assume that no matter which method you choose, I doubt very much if there are going to be any free methods, other than a BT but you would exlude many customers from this given your profession.
All of them will carry either a monthly fee and/or transaction fee. Have you checked out the rates for PayPal Business Account? It be worthwhile to look at payment company fees etc to see which would be more beneficial.
Check out this review site for online payment systems
http://www.reviewcentre.com/products4456.html
Cheers.