Large priority mail is too short, the medium boxes are actually longer at 13-5/8" x 11-7/8" x 3-3/8", and I still think that's not going to work for most keyboards. A Filco tenkeyless is 14", so you'd only be able to fit some pretty small keyboards in any of the free priority mail boxes. Really, I've got nothing. Maybe check at the local UPS/FedEx to see what size boxes they have? I think FedEx has an 18"x12"x3" that'd be about right...