I had the same experience that I posted about back in November... and was promptly dismissed by several follow-up posters whose local post office window clerks evidently ignore the statement printed on the front of every flat rate priority mail envelope as shown below.

The message clearly states; ..."with the adhesive provided as the means of closure"
This was explained to me on more than one occassion that the only means of closure is the adhesive strip that is provided inside the flap. Any other tape affixed (even priority tape) voids the flat rate and the package will be charged by the actual weight. Having said that, I've also had window clerks at a different post office apply additional priority tape on their own without me even asking.

So ... if some of you guys and gals are blessed with your local post office clerks who routinely ignore that printed message ... chalk one up for you! That's sometimes not the way it is at post offices nationwide.