Damage by USPS can happen even to well-packaged books. It is not common, though. 

• USPS is supposed to keep packages safe and dry, and they do a really good job - but nothing is perfect!

• Since no one can predict whether or not a package will suffer mishandling by USPS when passing through a rainy/snowy area and get wet, we do recommend (but we don't require) an inner plastic layer when Wrapping a shipment.

• If you live in a very rainy/snowy area or your mailbox is not very weatherproof, you can use Requestor Conditions to stipulate an inner layer of plastic for books mailed to you. Read more in How to Use Requestor Conditions.

Damage by USPS to a well-wrapped package is considered bad luck. The risk of this is split between the requestor and sender in the following way:

• if the book does NOT get to the requestor (= the requestor gets an empty wrapper or the book or wrapping is returned to the sender) the sender absorbs that blow and loses the credit
• if the book DOES get to the requestor (even if damaged), the requestor pays the credit for the book.

However, if the wrapping was truly inadequate for normal USPS wear and tear (such as the wrapping did not cover the entire book), and thus may have contributed to the USPS damage, then the requestor can ask the sender to refund credit, since the damage in that case would have been avoidable, and not simple bad luck. If the wrapper did enclose the book, and the damage was sustained anyway, that is considered damage from USPS, and the sender is not required to refund.

To read about marking an incoming book received with a problem, please see There is a problem with a book I received!.