When I sell handmade jewelry on Etsy and ebay, I use these bubble mailers from Canada Post.
These mailers are plastic inside and out and are self-sealing. The seal is very good, making these little packages almost completely waterproof. In addition, the plastic material is very durable, making the envelope tear-resistant and able to withstand a lot of abuse on its way through the mail system.
I most frequently use the smallest size, which is 10.8 cm x 17.8 cm (or 4 1/4" x 7"), but they come in numerous sizes. I have even seen some that were about two feet long. The small ones are $4 for 5 envelopes. This is more expensive than a typical bubble envelope, but the security of these against tampering, accidents, and moisture, make it worth it, in my opinion. I have never had a damaged parcel when using Canada Post Bubble Mailers!