In your situation you had to be led by whoever could deliver your parcel. That makes sense.
All I'm trying to say is that the cut-off points are at 1Kg (above which you can't use 2nd Class) and 2Kg (above which courier is cheaper, and above which only Rockerfella can afford Special Delivery). If there's anyway whatsoever to keep your parcel below either of those two thresholds, you'll save a good amount of money on postage. That's when the 20g polystyrene boxes often win out over 80-100g smashproof boxes.
That's all it comes down to.
I have nothing against the smashproof boxes at all. I just believe that there's a place for both types - sadly, suppliers tend to sell one or the other, not both.