Actually, the post office may have noticed, but they probably don't care, since I'm not really cheating them out of revenue; they just charge Fidelity as if the envelope were legitimate.

Only Fidelity might complain, but they created these postage-free envelopes so customers like me would send them deposits, so they probably don't care, either.

It was a just a fun thing to try, and great to see that it worked!