So do we have a consensus here that the soup in question was probably dried/dehydrated, in cardboard boxes, and this is what made them hard to pack?
Unlike someone else on this thread (ahem), I, too, was alive in 1963, and the only soup I remember from my early childhood was Campbell's, in cans; thus I was assuming that for some reason the Basque was claiming that it was difficult to pack cans of soup in boxes like the one we see on the side of one of the mules when the bear spooks them and they run off. And it made no sense to me that canned soup would be any more difficult to pack than canned beans.