Upload Files for Non-Profit Effort

Here is the situation: we are building a commemorative book for a church I work with. The idea is that people would pay for an advertising spot, and then upload what they would like to have in the book. We have looked at popular storefront providers (Shopify, etc) and while this support is available for a storefront, the cost is a bit too high. We can do a shopping cart with PayPal, but…how can we upload the purchased advertising logo to our site for inclusion in the book? The workflow is basically:
1) Sponsor buys advertising (in a particular size);
2) Sponsor uploads advertising (jpg);
3) Book committee retrieves uploaded advertising and processes it into the book OUTSIDE of the WordPress site.

Ideas are welcomed! I could probably write something to do this, but…we’re on a short timeline!

Thanks!