Everything charges per transaction. Credit cards are worse because you have to pay a monthly fee on top of that. If you have an iPhone, you can get a small swipe machine that will hook up to it and you can swipe cards from anywhere and have them digitally sign it on the iPhone.
If you are just starting out, I'd suggest something that doesn't charge a monthly until you can get some business going unless you are sure you can easily make up the difference. Nothing worse than starting a business and have to keep coming out of the pocket for upkeep.