Does anyone know of ways to integrate third party devices with modern and old cash registers?

Hey guys and girls,

I want to build a prototype of a device that integrates with cash registers.

I have a number of uses for it. It can be used for payments as well as 'punching' loyalty cards at the counter.

All it needs from the cash register is the price.

Does anyone have experience with this stuff?

It is important that the device be compatible with a range of cash registers.

I think since the cash registers can be connected to external printers they must be capable of communicating with other devices.

So perhaps it could send the price in the same way that it sends it to printer to be printed.

Any help is highly appreciated.