Add e-commerce to website with file downloads

I already have a website, need to add e-commerce capability. The site needs to accommodate large audio downloads (30 min or so each track), once payment through paypal is completed. Downloads will be be sold through "albums", that is up to twelve 30 min audio files and some pdf documents, or individual songs. System needs to be robust enough to handle slower internet connections and still do the job. Shopping cart should allow for user to add different products, remove them - if necessary, and then pay. After payment is done, automatic download starts. User to be able to return for X days and complete download, or download again. Ability to use discount codes in order. Discounts: *Create percentage based discounts *Create price based discounts *Edit discount code IN BID, PLEASE LET ME KNOW IF YOU ALREADY WORKED ON AN AUDIO DOWNLOAD SITE. Experience with this will be considered a plus.