I ship all my books with USPS, and with over 1000 books shipped I have only had one that didn't make it to its destination. That one turned out to be not USPS's fault; I stuck it in my mailbox on the day of a snowstorm last December, and the snowplow came by and hit my mailbox, knocking the book out into the snowbank. The customer called me after a month and said he hadn't received the book, so I shipped him a replacement and figured USPS just lost the first one. Then I found it about a month ago when the snow melted, about six feet from my mailbox LOL!
I ship most of my intake manifold adapters with UPS, and also have had no trouble. I just drop them off at the local UPS store already boxed and labeled and there has never been a problem. However, UPS charges a bunch to ship internationally, so I use the USPS for international shipments of the intake adapters. Prices are much better, and also no problems with deliveries. But shipping a large box with USPS in the US is more costly and time consuming than using UPS, so I stick with UPS for US shipments.
Bottom line for me is that I'm impressed with both organizations, and will continue to use them both - Jay