It can depend on who is sending the items. I've dealt with some US businesses that just use the largest standard size box from
USPS regardless of weight or size of the item being sent.
Looking at the USPS rates above, once you get over 4lbs, it's going to cost at least $43 for a medium flat rate box.
Have a look at
Auspost rates for the same maximum weight! Add $9 for the stupid security charge.
