I used these to mail calendars to people who ordered them from a non-profit where I am treasurer. They worked great until I tried putting two or three in the envelope (the envelope didn't seem overstuffed - they slid in easly). In a few cases the envelopes arrived torn with some or all of the contents missing. Next time I'll use a sturdier envelope for mailing multiple calendars.
The envelope is made of a nice quality paper. It is a little heavier but not enough to add weight to the required postage. Pull strip envelopes are the way to go no use of water or sponge is great. Size is true. I think the pull strip needs to be a little longer so there is a little more edge to grab when you are pulling it off.