So far as I can tell, the speakers are under UPS's maximum shipping weight (68kg), but heavy enough that they match up with UPS "special guidelines" for heavy packages (>31.5kg), which requires more sophisticated packaging, like a double-thickness cardboard box.

Conversely, I could call a commercial freight shipper and probably end up spending a lot more money, but then maybe they would do a whole shipping pallet, shrinkwrap and straps and such, which might maybe do better in a cross-country adventure?