The USPS® Flat Rate envelope is a great option when you have something that is over 16 oz but can fit in a 12-1/2" x 9-1/2" envelope. This label requires that you use the USPS 12-1/2" x 9-1/2" Priority Mail® Flat Rate Envelope (either padded or cardboard option). Padded envelopes and regular cardboard envelopes can be ordered for free from USPS or are often available at your local post office.
Just be sure that the envelope includes "Flat Rate Envelope" on it, like the one shown below:
Can you get this at the post office for free?