Based on the condition of way too many of the packages I've received, UPS folks also like to drive their 'Big Brown Trucks' right over people's boxes. And they charge too much for the "service".
In my experience, USPS generally provides faster, cheaper, more reliable service - but not always. About 5 years ago, I mailed two engineering text books, from Denver to a college student in Los Angeles. Because they were books, I sent them book rate. On the plus side, the postage was cheap, and tracking showed that the books arrived in L.A. in just three days. How cool is that?! On the minus side, some USPS employee then decided that taking the books the last couple of miles to the destination was too much work - and sent them all the way to Ohio, instead. It took USPS over two weeks to get that corrected, and actually deliver the package all of the way back in L.A. Tracking packages can be downright entertaining at times...
;>)
Your lucky they did not disappear for ever.
Don’t rely on “media mail” or book rate for fast service. USPS says up front 2-10 days for delivery. Also, such packages may be opened for inspection if they suspect regular merchandise is inside. If you don’t want to risk tampering, then don’t use book rate.
The last media mail package I shipped arrived in 7 days, PA to Utah.
For small light packages, USPS can’t be beat on price (1st class package rate).