The $13 USPS medium size box has a weight limit of about 65 lbs. Over that and they charge you a lot more. I was able to build a small wooden box inside of one made of scrap wood (paneling and 1x4s) and fit then in 60 1 lb ingots. I padded them with sawdust and was just 2 lbs under the limit, which is how I found out said limit. The postal clerk lady laughed and said that I was ripping off the Post Office.
Please check with the Post Office but that cost me only $13. Good luck.
This is the way I like to do it and line the box with some OSB top, bottom and sides.
