When I send my boxes they are always very heavy too, as long as all is packed well, there are no problems. Dont worry about the backs of the workers, as long as they can shift it, then they take it
I use a strapping machine from work to keep everything nice and tight, and I even (when I sent a really heavy box full of tools) went to B&Q and got them to cut me some 3mm hard board to the interior sizes of the box so I could line it for extra strength.
pic below of my final packed boxes:
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