I need a web store setting up using freeware (oscommerce or alternative)

I am looking to get a web store set up - likely to be very low volume sales, so not looking to spend a bucket load - initially at least.
I dont have a design, but it will be selling business software, so clean & professional.
Key features needed are:
Buy x get y free
Price matrix - single product, lots of variations (No. users, no. companies). These need to be selectable by the end user, and the price needs to update.
Digital Download (not required from the go, but must be able to be added).

We have a domain / hosting - for the moment at least it will be a sub-domain of our main site.