According to usps the large flat rate box(yeah the really thin flimsy cardboard ones) are good for 75 pounds. I bought 5 bags of shot 25lbs each off ebay. The seller put 2 bags in each flat rate box lulz. The one box with one bag showed up fine and on time. The one box with 2 bags showed up a week later torn to shit with a bag partially hanging out of it. The third box with 2 bags stopped tracking and went into limbo. Usps told me too bad so sad. Luckily the seller was super cool about it and refunded me the purchase price of the 2 bags. If usps says you can put 75 pounds in the box it seems to me its on them to deliver it intact in the box they designed and provide for that load.