We use three...

1. MIVA (we are a Premier Hosting partner)
Has a LOT of power and can handle a lot of traffic (Miva 5 is now using mySQL)

2. Vantagecart.com (built from PinnacleCart)
Inexpensive and has a lot of built in options that MIVA would charge extra for. Good for basic stores and if you are comfortable with some PHP you can really design a good site.

3. A secrect (http://vod.vantagecart.com)
The first Media Enabled shopping cart that you can also downlaod AND stream videos and music. You can also add DRM to the files if you want (optional). It is build from our relationship with PinnacleCart and the reason I've not been online much the last few months. You can sell real products, and also do download and streaming files. The site is 90% done but it will be finished by next week. We will also kick off a $97 (or less) DRM service (redappledrm) for the cart.

And..to top it all off..we offer a super deal for PayFlow Pro if you still need to get it.

Just drop me a note if you have any questions.
We have been doing e-commerce for YEARS and www.boysinc.com is built on MIVA platform.