I probably have a half dozen of those USPS game boxes sitting in my office. I even have some of USPS shoe boxes of which I am not sure if they still have them anymore.
Several years ago I made a batch of stainless steel washers for a marine guy in Idaho. I used the large flat rate boxes and cut apart some other USPS boxes for use as a double wall.
I used 3M 88 spray glue, the extra cardboard, and made some really stout boxes. I loaded 65 pounds of washers in each of those boxes since 70 pounds is the domestic limit.
You should have seen the look on the their faces when I showed up at the post office with 3 boxes and the counter guy couldn't even lift one of them. I sure got my money's worth out
of USPS on that transaction.
Tom