For the record, my
ebay letter did not have a stiffener. I used 3 sheets of 28 lb. paper to fold around the glassine. This makes the envelope thin enough to fit through their "slot test" and flexible, but protects the stamps from overzealous postal employees. Learned this from a German dealer who used to sell on
ebay. I feel like I am playing roulette at the local post office. About half of the clerks are knowledgeable, and the others are not.