they measure 10 1/2 x 7 1/4 inch flat.

the flap adds 3 inches, but that part get folded over and sealed.

they are new, not used.

the level of cardboard is flexible.

the are not super stiff or rigid, like you couldn't flex them, you can flex them.

they are sturdier than a paper envelope.

not sure what they are used to ship.

I did ship a book in one, but it wasn't a super fat book.

i have more sizes, if you want to combine shipping

this is a pack of 50 envelopes.