I sell a downloadable ebook that has 50 sub-publications (one for each U.S. state). Purchasers get the main ebook and one state e-book.
I'll be hosting the main ebook on my server due to the size of the download.
I doubt if you have something that will automatically attach the proper state pub the main, based on the purchaser's address. But is there a way I can offer the buyer a choice of which state ebook to add to his order at no charge, perhaps a dropdown box. It would have 50 choices.