The space they provide for that is, ostensibly, for both your "work" and your solution. They already have the answer checking/grading/tracking capability in Alcumus.
I have played around with one of the books a bit and find it to be preferable (for me) to the physical textbook. If you already own any of the AoPS books and still have the proof of purchase/receipts around in your email, you can purchase the ebook copy for $19 instead of the full cost.
As a courtesy to existing customers, we are permitting customers who purchased one of our books in the past to purchase access to the online version of the same book for the upgrade price of $19.
Please place an order for the online book at its regular retail price on our website. After you have completed your purchase, please email to orders@aops.com a copy of your AoPS sales receipt for the online book, along with a proof of purchase from your purchase of the paperback book (for example, a receipt from amazon.com if you purchased the book there). Upon receiving this documentation, we will issue a rebate for the difference between the retail price of the online book and the special upgrade price of $19. Please note that it may take several business days for us to process your rebate.
