What are you shipping, adult iguanas? I can fit two 5 ft. adult Mandarin ratsnakes plus heat pack in the 12x9x6 without much effort...

What if the only thing available "locally" to you was refrigerator boxes? Would you just use those? Pretending that options don't exist doesn't actually mean options don't exist. Order some boxes. Don't send us your business if you don't want to, order them from TSKSupply.com, they have many sizes. Or buy them from ULine.com and get insulation from Home Depot and insulate them yourself.