You seem to have this in mind, but the merchant account is a separate thing from the personal account. When you add it to your site you get a button for Paypal and then the ones for all the credit cards as well. So they process it all for you.

The primary advantage of Paypal, I think, is that a lot of people will click through on Paypal that wouldn't buy otherwise if they are concerned about security. There are some people who express having had problems with Paypal but I haven't and most people like it. I don't think many people will recognize most of these other programs. You pay for the name and sense of security, mostly, so I'm not sure trying to find something cheaper is worth it.

James