I have shipped a HEAP of cast boolits (as Precision Machined Parts) through the USPO and their Flat Rate Box (FRB), whichever size, is limited to 70 pounds.
The Small FRB measures (inside) 8 ⅝” x 5 ⅜” x 1 ⅝”. The Small FRB box is 75.33 cubic inches (ci) of shipping space, which if loaded to capacity with lead (0.4098 pounds per cubic inch) would weigh 30.9 pounds.
The Medium FRB is either 514.25 ci top loading or 546 ci side loading (both about 7X the capacity of an SFRB). The Large FRB is 792 ci and 10X the SFRB. Either Medium or Large FRB would be much more than the 70-pound limit if loaded with lead.