It may not cost more to use a box than using an envelope. Even if using an envelope, you can't mail it for the price of a letter because of the thickness. But even using a box, as long as it's below 13oz., it will be first class mail and only a few oz. heavier than the envelope. I save all small cans and yixing boxes and use them as mailer boxes
Also there is a type of mailer (often used by media mail) that has thickness on two ends but not on the two long sides. It provide some thickness protection, but the mailers themselves would cost some money and it's not perfect protection anyway.