bookseller donating unsold books to Goodwill

We sell used books online, and we donate books that don't sell to Goodwill. We haven't been getting receipts from Goodwill, but I can make a list of which books we donated. What's the maximum amount that I can put down as a donation if I don't have the Goodwill receipt? In the future, are there any special rules for donating to Goodwill? If I donate 20 books and put the donation amount as $40, and then put that on the receipt they give me, is that good enough?