e-Junkie provides a neat and comprehensive shopping cart and product management solution, including the ability to sell downloads and to sell through affiliates. We're fully integrating e-Junkie with the Hubspace products module. (We're not so keen on the name, but your customers don't see that).
Included with: Interactive plan - £18 per month
3rd party charges: Approx. £3.50 per month for up to ten products (no limit on transactions, no charge per transaction). Subscription levels go up based on the number of products. (As in all cases with e-commerce, you will also require a merchant account and payment gateway, or you can use Paypal Payments Standard or Google Checkout, which will each charge a percentage on transactions).
More information on e-Junkie from their site...