Just remember... The US Postal Service is a private company which has an exclusive contract to provide service to the US government.
While it is a crime to tamper with the mail, it is not a federal offense to take an unused envelope from the post office, especially when those envelopes are offered for people when using the post office's services.
It would be no more a crime to use a Tyvek envelope from Fed-Ex.
If you took one or two envelopes, I don't think anybody would care. If you took a whole stack of them, somebody might call you on it.