Have considered using a shopping cart? See
this thread.
I have used Cubecart to do exactly what you want to do on one
website hosting and it seems to be holding up quite well. I have not had the opportunity to try any others yet tho and judging by the result of the poll in that thread there may well be a better solution.
I would be lying if I said it didnt take me a while to get my head around cube cart templates too.