If Amazon already sent the books and they came back, the issue is almost certainly the facility's policy rather than the retailer. Amazon is the standard that most prisons accept for book orders because it ships directly from a recognized retailer with a packing slip, which is what mailrooms require. If Amazon books are being returned, it is a strong signal that Florence Correctional Center has either changed its book policy, restricted the types of books allowed, or moved to a system where books must come through the facility's own approved vendor rather than outside retailers.
Before trying a different store, call the facility directly and ask what their current book policy is. Ask specifically whether books from outside vendors are accepted at all, which retailers are approved if any, and whether there are restrictions on format, genre, or content. That one call saves you from sending something else that gets returned.
If books are off the table entirely, magazines are a practical alternative that most facilities continue to accept. A subscription keeps something arriving regularly rather than a one-time delivery, which means your friend has something to look forward to on a consistent basis.
InmateAid offers magazine subscriptions across a wide range of interests and is currently offering a third off all subscriptions. Send an email to aid@inmateaid.com to get a coupon code before you order. It is worth taking advantage of if you are going to set up a subscription anyway.