I recently launched a niche startup selling addon content for Photoshop. It's a purely digital-goods business: people pay money and in return they get an email with a download link.

I looked at a bunch of solutions (shopify, 2checkout, paypal, authorize.net, e-junkie, and others). Ultimately, I went with a company called FastSpring. It's a third party shopping cart solution, but they let you use your own xhtml/css template. They do include some text in the footer to let you know who's providing the service, but it's not really intrusive.

If you want to see how it looks, here's a page from my site. Click the add-to-cart button: http://photoshoplayerstyles.com/sale

Overall, I really like their service. They pay out twice a month, directly into my bank account. I'm in Canada, and most places just won't do this, so it was a nice surprise when it just worked.

Edit: I should mention, they handle international sales smoothly, too. I've had sales from all over the EU. They detect the country, do currency conversion, charge the correct VAT, etc.