Flint

Flint is hands down the best mobile payments app in existence. It is very easy to scan cards, way better than the magnetic card reader that took 5-10 trys to work most of the time. I like that I can send email receipts to customers for cash and check transactions in the field and not have to do additional data entry when I'm back in the office later. The invoicing works well but make sure you're set with the required credit limit before you go sending out invoices for amounts that your customers can't pay because of your credit limit. I just sent in my drivers licence and a statement and my limit was raised within 12 hours.