I occasionally sell children's clothing on eBay. When searching for a durable, inexpensive mailer, I found the Scotch Bubble Mailers.
I like these mailers because they have a layer of bubble wrap lining the inside of them, giving a bit of protection to the goods inside. To seal them all you have to do is pull the covering off the adhesive strip and push the sticky part against the other side of the envelope- no licking required!
I was surprised at how much you can shove into one of these things! I occasionally have to put a piece of tape around the sealed flap if there is too much stuff inside, but that is more for my peace of mind than any issues I've had.
The only drawback of these mailers is they have a To: sqaure pre-printed on them. It likely doesn't matter, but I find myself frequently having to extend my writing beyond the confines of the box. If you're not as anal as me, this likely won't bother you.
If you purchase these singly they are much more expensive than if you purchase them in bulk at a store like Sam's. In fact, if you need more than four or five, it will probably cost you just as much to buy gox of 24.
I love these mailers to ship things in! Between the size 2 and size 5 mailers, these are all I use. I highly recommend them.