The adhesive strip is pathetic. Will not hold the flap shut - need to use tape to close the letter. It is also not completely across the flap. These are cheap and not worth it.
This boxed set of 100 envelopes are in fact made of paper. The are almost but not quite excessively thin. The best part is the self stick. If you take the envelope flap and stick it to another envelope flap it will stick very nicely. If you close the flap onto the envelope where it rightly belongs and miss align it no problem you're saved. Just touch the seal and it will peel right off without leaving unsightly mess. I'm not sure how it will fair in a high speed USPS sorting machine.