Drop down menu & digital files

I have successfully added a drop down menu to my site but am struggling to figure out how I could have it retreive different zip files based on the selection made. As of right now, it is set up as an ecommerce site which allows you to choose an item and pay for it after which you will receive a link to the dowload you have purchased.

What I want to do is be able to sell multiple size downloads for different prices using a drop down menu. However, I can not figure out how to retrieve the appropriate zip file. Any help would be appreciated.