Yes, Jim:
¶ "I believe they impose the thickness requirement so carriers don't have to look at every thin piece of mail to determine whether they need to scan it." Bingo! I have long thought that, and I never saw it mentioned anyplace else.
¶ I keep a small bag of Styrofoam peanuts near my envelopes. I throw in one whenever I think I may be below 3/4" thickness.

Robt.