My father does this... incessantly. He has stacks of stamps he uses to send out for work and he gets them all at major discount on ebay.

It turns out the post office, mostly doesn't actually care to count up the 1 and 2 cent stamps.

There's a group of hobbyists committing felonies out there making their own stamps and sending them out. The post office doesn't notice.

One sillier trick was, forget a stamp, it'll get returned to sender. Simply put your destination as the return and you get a free mailer.