Would it be possible to include Square payments addon? They have all the information on their API here.
Square is becoming very popular, and many shopping carts such as Ecwid, are now including their payment API.
In fact, we have the Beta version for Square up payment plugin. But it is quite difficult to setup it (Both customer's site and OSB side). We are still working on beta version. When it is complete, we will upload the payment plugin for OS Services Booking.