The US Post Office will give you large flat rate "game board" boxes, but you have to order them online, they don't keep them in brick and mortar POs.
The come in packs of 25 (so if you order "3" you will get 75) and are delivered to your front door in a few days.
They cost about $18 to ship anywhere in the US (I had an M delivered from Atlanta to Alaska in under 48 hours once, but don't count on it) and if you print postage through ebay/Paypal it is discounted to about $16.
Same box international goes for about $85.