Just remember there is a weight limit, 60 pounds. I have a reloader on my route, I hate it when he get overtime. One week, cases like 5 of the big flat rate boxes, next week lead in the same size box. Almost rip my arms off if I don't check the address first. If you go to USPS.com you can order boxes for free.
PS For your and the other persons mailman right HEAVY on the outside LOL