Hi Gary,
I personally do business through Clickbank, 2CO.com, and PayPal. As Lynn mentioned, each has its own benefits and down sides.
In regards to your question, though, I am not sure if 2Checkout allows multiple sites (I only have one through them), but the account really has no limits as to how many
products you can set up in your account.
The $49 setup fee is a ONE-TIME fee.
Not per-product.
The company does, of course, take a certain percentage of each transaction sale, however. I believe it is $0.45 per sale, plus 5.5% of the sale amount. But that's the only other way that you pay them, after the initial setup fee.
I would look at the 2Checkout FAQ page for answers to the 'multiple site' question - .
I hope that helps you out...let me know if you have any other specific questions!