I have never ordered something online, it be shipped through USPS, and it come on time. They also can't ever keep the packages tracked, so you never know where your items are.
I never have any problems with UPS or Fedex, but USPS? They are complete garbage. I would rather contact a seller and pay them to ship my orders through UPS or Fedex.
That way I'd know where my packages are at all times and my packages will always come on the scheduled delivery date. I have had a package come nearly a week after the scheduled delivery date with USPS.