With the recent changes with the bank interchange fees, you need to review your surcharges on credit/debit cards if you impose them. As to calculate what you can charge is not a simple matter like some costs are acceptable such as merchant service fees and terminal fees there are many others that are not acceptable. So I do suggest that you check with your EFTpos provider first and then document your findings in your diary before changing your rate in your point of sale software. I think too you should discuss with them the new situation with American Express (AMEX).
I think to help you will find this page useful.
