Dude, USPS, as in the U.S. Postal Service, is absolutely reliable. If someone is seriously sending you stuff and you are not getting it, you need to look out!
Many people in my family work for USPS. A member of my immediate family is a postmaster. I can guarantee you that the ol' "must have got lost in the mail" trick is still one of the biggest cons around.
I can tell you that to my knowledge I have never in my whole life had a single piece of mail "lost in the mail".
Once in a great great while you will hear about some mail carrier stealing something, but the ones that do are total psychos. Postal workers are given merit tests before they are hired, and are generally really intelligent people who often lack college degrees, and there are few other places that they could earn the type of living that they do which includes many benefits and retirement at age 55, so they're generally reluctant to risk their jobs (and a stiff federal prison term) looking through boxes and boxes of people's junk to find that one "real cool thing to steal".
Of the very few psychos that do steal, they're generally looking for checks or cash or things like that and not packages.
If someone truly dropped packages off with the USPS and you haven't got them by now but should have, then I think you may have a major problem on your hands.
I have to say that either you are getting screwed by a supplier or targeted by an agency, one or the other.