I've gotten a decent number of envelopes where the flap is simply tucked into the envelope rather than any attempt being made to seal it. It's all been junk so

I've gotten a decent number of envelopes where the flap is simply tucked into the envelope rather than any attempt being made to seal it. It's all been junk so if a few come apart they don't care. I believe there was some rate advantage to doing it that way.