The U.S. Postal Service demands that I discard perfectly good, used Tyvek Priority and Express Mail envelopes, and I am tired of it. Their concern seems to be that people will grab these envelopes ...

We asked the United States Postal Service and learned Priority envelopes must be used for priority shipping. You are not allowed to black out and reuse the envelopes for regular mail.

Keep in mind this legal caveat from ITS: This project also warrants a disclaimer: Priority Mail envelopes, whether new or used, are property of the United States Postal Service.