I was very surprised to hear that USPS was involved in some of these being shipped to customers. They are notoriously slow and unreliable with personal mail so I'm surprised they would have played any role in the initial launch. This reeks of a cost-cutting measure by AT&T.
I have often heard people complaining about the USPS being unreliable with mail but I have to tell you that is not my experience. We have several newspapers that use USPS deliver on weekdays (carriers on Sundays and holidays). We get same day delivery at 2nd class rates. My iPhone 6 Plus was mailed and it had excellent tracking. It was delivered on time just as it was supposed to be.