I don't mail many things anymore. Between paying my bills online and staying in touch with friends and family through email, it is just not necessary. But when I do use "snail" mail I like it to be secure and private.
I like the Evidence envelopes by Ampad for 2 reasons. First, they are lined so no one can see through the envelope even when held up to a light. Second, they are pull and seal. This is important because I am not going to lick an envelope to seal it. With these envelopes licking is not necessary. You just pull off the protective strip and press to seal.
The only complaint I have with these envelopes is that every once in a while, the glue will be partially dried out and the envelope will not seal properly. I just use scotch tape to make it secure.
These are the #10 or business size envelopes. They measure 4 1/8 in. x 9 1/2 in. and come in a box of 40 for $1.19.