I had a similar incident when I bought some small engine parts, total was about $100 and the seller shipped them in one of those nylon/fiber bags and it came open ... I was delivered an empty bag ... The seller said said it had happened before. I asked why he didn't ship in the free Priority Mail boxes ... Said he didn't know about them and he shipped me replacement parts in them ...
The only other problem I've had with USPS, recently I bought 1,000 loose packed 9mm 147's ... like the OP's in a plastic bag, in a small Priority Mail box ... by the time I received the package the cardboard didn't feel anything like cardboard but fortunately it was wrapped in multiple layers of clear shipping tape ... I received mine intact, however the box felt more like a 27 pound water ballon.
It's the USPS rules (if it fits, it ships) but with that kind of weight the box wasn't designed for, it's up to the shipper to make sure it gets there intact. I bought my "lot" with shipping included and had anything happened to it, I would have expected the seller to make it right as he didn't offer insurance options ... Fortunately mine came through without incident but as with many things you get what you pay for and something like this, insurance would be a good idea or accept your losses.