Ask 37signals: How do you process credit cards?
At my work, we were pissed off at effortlessE, whose POS we were using. In about a week and a half, during the Christmas season, two and a half developers wrote us a new one. It's been running for three weeks now with barely a hitch, and it's much better than the old one.
I was surprised at how easy it was.
The biggest challenge is when you are from outside of the US, and on top of that, from a developing country that is frowned upon.
My country isn't even supported by PayPal. Now that sucks. Let alone almost every merchant account provider in the US. And we were rejected by an "international merchant account provider" with a "100% acceptance guarantee".
Oh well, all these things will never make sense to someone from the US, where you can actually take things for granted.
You could consider invoicing via Amazon FPS or YC's own TextPayMe ( https://textpayme.amazon.com/sdui/sdui/index.htm )
this is old, boring and obvious news/content.
I get clients online monthly with payment processors and live payment gateways and explain it to them a lot clearer and more concise than this article (and they arent web geeks that read 37signals blog).
Good rule; If it isn't Auth.net or Verisign it probably sucks.
activemerchant ftw!