I have done a bunch of research and I have found Quantum Gateway to be a very competitive solution. I have also worked with Evalon and Usight (offered through costco) and they are just a little bit cheaper, but much more difficult to integrate. I have also worked with PayPal and it was quite a bit more expensive ($30/month) and can be confusing to users. Beware of some others that have huge early termination fees, large monthly minimums, setup fees and etc.