Hi Sarge,
You can easily put in 6 full racks or 300 strikes(maybe more) into a Medium Flat Rate Box, but I have to agree with Ed Janich that you must double the box(line the outer box with another box)
because it is very heavy and will bust open at the seams or corners. I received a shipment and the seller used the white casino racks stacked, bubblewrapped, and taped up with duck tape.
Received everything okay. I received from different shipment from another seller and the corners had busted open. Fortunately, the rolls of strikes did not fall out of the bubblewrap inner bag.
Picture shipping a dumbbell weight, whether or not it can break through the cardboard box being thrown and bounced.
I hope you are well and this helps,
Gene