I've sold a ton of stuff through Amazon. There's basically no way to "beat" people who will sell at a lower price than you. If you sell a book for one penny, yeah, the shipping credit might leave you a little left over, but it's Amazon and not you that determines what the shipping credit is. For some larger books, I've actually had to pay more for shipping than the credit provided.
Just pick the books that have a decent going rate on AZ and sell those. Take the cheap ones, and do them as a lot on eBay or trade them at a local bookseller.