"If you seal an envelope and realize you forgot to include something inside, place it inside the freezer for an hour or two & it'll unseal easily."
Before anyone asks, yes I tested this out. On several different envelopes, in fact. I know there are different kinds that feature different glue, some you have to lick, some are self stick, some use thicker paper. Needless to say it didn't work at all. I'm not surprised, if it worked wouldn't we all run the risk of our mail popping open during the winter months? I tried this with newly sealed envelopes as well as various ones sent to me, and they all tore like regular unfrozen envelopes when I tried to ease them open. It seems most sites that recommend this haven't even tested it, and the one youtube video I found had the guy opening it, but he only sealed the most bottom tip of the envelope, which would be fairly easy to open gently without even freezing it.
Honestly if you DO forget to add something to an envelope, and you've already added the stamp (and don't want to waste that by having to put everything into a new envelope) the easiest thing to do is to use a sharp knife, open it from the bottom carely, add what you need to add, then seal back up with tape. Not ideal, but it works far better than trying to pry open the seal.