I've been using the flat rate envelopes like I always have... stuffed and blulging to where I can't put anything else in it... and I haven't been refused lately. I put tape on all the seams, the flap, and reinforce the bottom too. They do not want tape over the bar code on the shipping label, but as far as I know that's the only place you can't put tape.
That being said... I did have one substitute clerk tell me about a year ago that I couldn't put tape anywhere on the flate rate envelopes so I politely said "thanks, I'll wait for the next clerk." The next clerk didn't blink an eye, he just scanned my packages and gave me a receipt.