I have had several things disappear into the "Black Hole" of the Richmond sorting pit.  Had a scope stay there a month, wife had some family heirlooms stay there about as long.  Both were cheapest USPS shipping.

If you do a search, it is a well known problem for USPS.  I suspect it happens when the cheapest USPS mailing is used.  What used to be call Parcel Post of something, something else now. 

First class/Priority may be more but it does keep moving. 

Standard Post is how all of them have been shipped so far and the shipping, handling, and insurance runs about $50. If you go to the next cheapest option it is Priority Mail 3-Day. That looks to be just over $75. Both are costly, but still less expensive than UPS.

I don't ship stuff regularly anymore so I'm not sure that standard mail works the same way.  But when it was called parcel post the catch to the price break was it's on a time and space available basis.  So while they could also be waiting for full containers they process everything else first anyway.  Express, priority, and first-class mail then parcel post.  Shipping times to Hawaii from the east coast were often 4-6 weeks and 2-4 weeks from the west coast.  I think the only thing slower then parcel post was book rate now media mail.  It's processed after parcel post.

I think something similar is true for the USPS tracking.  It used to be called delivery confirmation now it's just tracking.  Workers were not obligated to scan it between acceptance and delivery.  And often you would end up with gaps in between sorting facilities and delivery.

It's always been worth shipping priority for me.  Save a bit on UPS or FedEX and receive it in a more timely fashion.  Even priority if it misses a few connections can take up to 7-10 days from the mainland.