My bad! I was looking at a regular, plain, large envelope, used for First class postage - NOT a Flat Rate envelope.
I did see where it says if it fits it ships and it can weigh up to 4 pounds for a F.R envelope. I would print the information right off the website, take it to the post office and ask them to show your their rule book. Speak to the Post Master. I do get tired of rules not being applied uniformly.