sometimes FedEx has their branded Express triangle boxes. They are free (just make some shit up that you have some marketing material that you need to ship to 3-4 locations if they pitch a fit). They are flat and I fold it up inside-out so it's plain brown on the outside.
Also, use PirateShip. You get a slight discount on shipping most of the time.
For example, I recently sent a putter in my "fedex" box, USPS priority for $7.65 to KY.