First, you may wish to acquaint yourself with the way our built-in download delivery feature normally works:
Note that you can allow up to 9 Attempts on any download link we issue, and if you leave the Hours setting blank for the product, the number of Attempts you allow can be used at any time indefinitely until they're all used up, then the link would expire. You can also re-activate expired links manually in your Seller Admin.
This help page explains how to sell gift coupons; however, coupon expiration can only be set to a fixed calendar date, rather than some arbitrary span of time after the code is issued to each buyer:
Many musicians using our service will simply set up the series of unique, single-use discount codes explained there, but without creating a corresponding gift coupon product to sell those codes online, so they can use the codes to sell downloads in-person at live shows. Each buyer at the venue is handed a code for their purchases, which they can use in their cart at the musician's site to obtain their paid-for download items at no additional cost. We've even heard of one musician selling vinyl records with a code written on the sleeve that the buyer can use to order a high-quality digital download version of the same music at no extra cost.
We don't have any built-in way to administer memberships or subscriptions for unlimited access to a complete library of downloads, not least due to the high potential for link/login/code sharing and product theft. Even if you just sold each buyer a code good for 100% off any order and reusable indefinitely until some fixed date, there'd be nothing to prevent them from sharing that code with others, posting it to public coupon-sharing sites like RetailMeNot, etc.