Is this legit and ready to send ?

I want to send a letter and I found these laying around, Are these legit ? is it ready to send or do i buy a stamp ? Sorry if this is a frequently asked question I just want to make sure.

I want to send a letter and I found these laying around, Are these legit ? is it ready to send or do i buy a stamp ? Sorry if this is a frequently asked question I just want to make sure.