If a book is received damaged due to poor wrapping, how do I mark it??? Damaged by sender or post office?

The book was wrapped in plastic, but then covered with a single sheet of printer paper ( the book was a trade size)

the paper wrap only covered about 1/2 of the back of the book. So the plastic and of course the back cover were showing.  The book was kinda bent and had a slice in the back cover

If it was poorly wrapped, and the wrapping contributed to the damage, then the sender is partly to blame.  From the Help Center:

"Damaged by USPS

• You will have the option to describe the book condition problem in a text box - the text you type in will be included in an email, and sent as a Personal Message, to the sender.
• You will have the option to request a credit refund - this is UNchecked by default for this problem type.  If you check this option (if you believe the Wrapping contributed to the book's damage by USPS), the email the sender gets will include the request for a credit refund. If you leave this unchecked,  the email sent to the sender will NOT mention or request a credit refund."