We have a small USPS here and she is the one who tapes it so it wont' get damaged in the mail. I mailed some yesterday and no problems. But I have heard, even when your PO accepts it, other PO's can return it for postage. Grrrrrrrr
You can get online at USPS.com and order the new flat rate 4.95 envelopes that have strong tape and are like those bubbly ones and they close better. Who knows. ONe day it's this way and the next not.... You have to tape the boxes that are large flats since they have no sticky. They are silly. I do love the medium tall flat rates with the sticky on each end. Those really work well, no tape needed there. Have you ever tried to open those? Really tough.
Like everything else, if we want to use their services, we gotta play by the rules. YOu can go onto USPS.com and find those too. Make a copy and take it with you.