shopping cart software - please recommend.

Hey gang,
Can anyone recommend any good shopping cart software, please?

I need something that
- I can buy, one off, that will do it all
- I can easily add to a site (and easily understand)
- Does not need a monthly fee or any costs to continue

I need something that will cope with probably only tens of products to begin with, but something that can grow as the product range increases, but at most possibly into the hundreds.

Does the software take care of dynamic pages of search returns, or product ranges? Or do they need to be built some other way?

Essentially is shopping cart software all that’s needed?

(Payment methods are being looked into separately)

I am not necessarily looking for the cheapest, so any recommendations would be useful please.

Thanks very much for any suggestions, gang.