You can get away with posting small clothing items as letters as long as you pack them FLAT.

When I do it, I use an old cereal box cut down to fit the envelope, use the white non-marking tape to tape the clothing down onto one piece of card, cover it with another and tape the edges of the card down, then slide it into a large envelope... this way the clothing item won't bunch up inside and the postage will be under $3.

It's alot of effort though, and if you sell alot of things it would be very time consuming.