I use tape to reinforce because a lot of the older envelopes seem to lose their sticky on the edges. I always thought that warning was because people used flat rate envelopes to mail items that were too thick or wide, and the regular flap could no longer reach. I used to ship boxes packed inside that envelope!

I'll check for the current stock next time I visit the P.O. I don't use this service very often, so I have older envelopes on my shelf at home.