My understanding is that the entire envelope must be able to flex if it is going to go through the automatic letter sorters. Otherwise, you could put a flexible part on any item you send and argue it should go envelope rate.

If part of it can't flex, the entire envelope can't flex. That makes it rigid in the post office's eyes, and therefore a package. I always pay $1.71.

If you use a bubble envelope with something that is flexible, such as a slot card, it may lead to a different result.

I do not advise the post office, though, so it is up to its managers and employees to interpret their own rules. If anyone plans to argue with them, I have pointed out the appropriate regulations.

The only way this can be resolved nationwide is for the bubble envelope manufacturers to lobby the post office to clarify its rules (not very likely, if you ask me). Until then, we have to take the rules as we find them.