Hint from someone who has literally shipped two tons of lead.
weigh out 65 lbs of alloy.
get a med flat rate box.
get six small flat rate boxes.
use elmers glue to glue the bottom of the MFRB, then tape the bottom closed too.
make up the 6 SMFRB's, tape them up after adding the alloy, use newspaper to fill the gaps so it doesn't rattle.
tape sides of MFRB, then, amazingly add 6 SMFRB's inside the MFRB.
glue the top closed and glue it to the boxes inside, then tape the top.
label as EXTRA HEAVY, add shipping label
done.