I recommend Fastspring (http://www.fastspring.com). Yes, it's more expensive than others but it has a lot of features that make it worth it.
The main selling point for us was that essential you sell your product wholesale to Fastspring and they become the seller. This makes accounting and taxes much easier, especially for EU customers.
I can also vouch for Fastspring. The reseller part is really important. It means they take care of billing and VAT, similar to the App Store. It's a bit more effort to set up compared to the Mac App Store, but they take less and pay out faster. Customer support is really good. Their API is a bit annoying to work with, but gets the job done, and the customer support makes up for unclear docs. I'm really happy with them.
I'm not in the same boat as OP, but I definitely can vouch for Fastspring in general. It's extremely convenient from the business standpoint that I don't have to worry about all of the complex international tax laws, etc.
Also, I've used PayPal API, and found the Fastspring API comparatively easier to use once you figure it out.
Love FastSpring. Easy store manager and great tech support. Plus, I don't have to worry about anything because they handle all the invoices, VAT and other boring stuff.
Fastspring is great, I've used them for the last 5 years. They handle VAT as well, one of the main reasons I use them (I'd rather not deal with that bureaucracy myself).
It's still a good idea to have a second payment processor, in case one has downtime or a customer has a problem charging their card. I've never had problems with Fastspring, but I occasionally had problems with others - I have a switch in my backend where I can flip to a second payment processor instantly.
