Already done (no ish)
USBSwiper has created a POS (point-of-sale) application using Filemaker. A merchant must have a PayPal account and be approved for Website Payments Pro and Virtual Terminal services offered by PayPal. Once that's done, USBSwiper will sell or rent the application to a merchant. Rental is designed for small merchants who need a trade show payment system or who might be a seasonal business, purchasing the application allows a web merchant to process mail and phone orders much easier than on most e-commerce shopping cart systems.
This is not second hand knowledge. I now use USBSwiper and have used it at a trade show on my laptop with a Verizon wireless connection. It worked perfectly. From an administrative perspective, I've never had a merchant credit card account that was so easy to use. From an economic perspective, PayPal charges a flat rate for all transactions - 2.9% of the sale plus .30 per transaction. There is a $30 monthly fee which can be turned off anytime if the merchant is closed for the season, on vacation, or any reason.
Most consumers understand that the merchant pays for the acceptance of credit cards; what they don't understand is that their affinity card or rewards card costs the merchant more per transaction than a standard credit card, or what is called a "qualified" card. All non-qualified cards are charged more. Want to take a guess how many cards are now deemed "qualified"? Out of every 100 cards swiped, maybe 10 to 15 are qualified, which means they are charged at the lowest discount rate. All others are non-qualified and cost the merchant more. Add on fees such as statement fee, "abuse of system" fees (it's real) and anything else the card processors can cook up and the real cost of card acceptance can be as high as 6% - 8% or more.
PayPal's venture into POS applications started with the development of their virtual terminal application, but only the smallest of retail merchants could use it. It just doesn't work in a high traffic retial environment. The USBSwiper application makes things easier, but the flat rate card acceptance fees is what makes it work for me. All other processors can go to hell as far as I'm concerned. Paypal is our choice for card acceptance and will be as long as they don't screw up or get greedy.