From what I've been told the envelope must be able to close of its own accord (Like, you can't cut the flaps and retape them to make it work for you), and you may reinforce the seal with a single strip of tape, as long as it doesn't wrap around to the front.
Now, Flat Rate BOXES also say you may reinforce the tabs and flaps, I've never seen anything about reinforcing the bottom flap on the FRE. Never had one bust, but it *is* a sealed opening, you know?