You can go online to the US Post Office and order "game board" flat rate Priority Mail boxes. They are not available in brick and Mortar POs, but will be delivered to your doorstep free of charge in boxes of 25 (ie if you order "3" you will get 75!).
They are large, big enough to hold a well-wrapped 122, but will easily fit 2 standard keyboards, maybe 3 if they are thin.