I've had stuff arrived torn to bits from UPS, Fed-Ex & the USPS. I've even had Fed-Ex turn stuff around & send it back to the original shipper twice on me which has caused me to dislike Fed-Ex immensely. My wife works for the post office & I can tell you most of them are very careful around here, but I know of at least one instance where a lady picked up some mail & went driving off with the back door open & mail was flying out all over the street!

The post office is usually pretty serious about keeping stuff straight though & having stuff stolen from your box then resealed with books in it is unreal. I'd take it to the post office & report it a.s.a.p. regardless of how long it has been. They know who has handled that box if it was handled properly & they take a very dim view of thievery.  Just my $.02