Digital River has a huge variety of solutions for software authors. One of them should fit the bill pretty well.
I've used Share-It, RegNow and I forget what else -- they all seemed pretty good.
Avangate is another to check out.
There are many more as well -- Cleverbridge has a client, which is nice. (Or they had a client last I looked.)
Northstar (Northwood?) is a smaller one, but the owner, Jeff, really goes out of his way to help people.
I keep hearing horrible things about Plimus - they seem to go down a lot. But, they're also very popular.
PayPal is one of the most versatile, but you really need to do a LOT of work to get it done right.