I have received pkgs that have been sent UPS and then through the PO . There are sometimes I don't get the pkg in a timely manner. I order supplements by mail and have had to reorder at least once. Packages get lost . We have a mailman that doesn't like to get out of the truck to put packages on the porch so stuffs them in the mailbox. We have one of the ones that lock so where he can put the mail in is very small.
That being said I do ship packages by USPS simply because for me its cheaper than UPS or FedEX . I do it online and have never had a problem. Most times I don't use flat rate because I can send by priority by weight cheaper. I use my own envelopes and boxes for the most part or the not flat rate boxes the PO has. Rarely is flat rate cheaper. And if they are going up again!!
And the taping on of the label to make sure things are sealed is exactly what I do for the most part.
