Probably mixing up double-corrugated with double-boxed. The # of corrugations used makes the box stronger. You can put a ton of weight into a triple-corrugated box if the inner packing holds it firmly in place and prevents stress being concentrated in small areas - which applies both directions, meaning you want to prevent the heavy item from chewing its way OUT of the packing, just as you want to prevent external impact from focusing the energy of the impact on small areas of what's inside the box.
I am going to go out on a limb here and suggest that a particular kind of comment prevalent in this thread be considered by the moderators for inclusion on the list of "Topics that have been beaten to death, and we're done with them". Specifically, the statement that "X freight carrier or Y mail service or Z freight carrier is the worst". Literally EVERY thread that speaks about shipping damage will contain one or more such assertions, always based on that particular member's personal experience (at least, the experiences they remember now or that hurt the most).
And, virtually every one of these threads also has such assertions that name every one of the commonly available domestic services i.e. UPS, USPS, Fedex. Then, almost invariably someone else (presumably that understands the logical futility of those collected statements) points out that it isn't who you ship with, but how you pack the item in question, that mostly determines whether it gets delivered in one piece. I happen to believe that personally but it has become so repetitive and so widely ignored by the UPS haters, the USPS haters, and the Fedex haters here on AK that it's also become a waste of time posting it.
Just a suggestion, ymmv.
John
