From time immemorial envelopes have had a flap that seals on the back, and stamps and an address that go on the front. Fedex and UPS respect this tradition.
But the USPS makes envelopes like the one in the picture at left.
What looks like the front in the top part of the picture, is actually the back.
What looks like the back in the bottom part of the picture, is actually the front, where the stamps, address, and return address are supposed to go.
I discovered this the hard way yesterday.
Eventually the postal supervisor scrawled a more or less illegibly cursive “ADDRESS ON BACK” on what I thought was the back of my Express Mail envelope.
The post office has wisely printed “EMS” on what I thought was the front of my envelope, an abbreviation you usually see on ambulances.