Interfacing your credit card payments into you cash register is not only a good idea but it is very affordable these days. The number one reason to interface your cash register and credit card payments to to avoid mistakes in tendering sales amounts in the credit card terminal after the cash register. As an example lets say you ring up a sale on your cash register and it totals $19.49 and your customer hands you a credit card for payment. Know you have to re-enter the $19.49 into the credit card machine. Problem is that is is very easy while in a hurry to enter that amount as $1.49 or 9.49 or $194.90 and in the rush no one notices. Don't believe me? Well then go back and match the sales in your cash register that were paid to the charge button to the amounts on your credit card machine. If you do any volume at all you will find some glaring mistakes. The larger your average ticket the larger your mistakes.
So when you interface your credit card payments in your cash register you can not short change the cash register. The register automatically inserts the sale amount the credit card is be to be authorized for. Yes it is possible for you to enter an amount less then the amount and the cash register will let you do that, BUT the cash register still won't completely finalize the sale until the amount is paid for in total whether by cash, or credit. This keeps your clerks from making those mistakes or at worst they notice right then so they can fix it.
So here is a great source for SAM4s Cash Registers that do credit card interface and here is a link to Datacap Systems who make the credit card interface device that works in SAM4s Cash Registers as well as Sharp and Casio Cash Registers