You're labouring under the false premise that anything except your smashproof box must lead to breakages. As I've not had any breakages when sending 10oz or China mugs in polystyrene boxes, it doesn't factor into the cost. Nor do I use any "outers". The only thing that factors into the cost is weight.
I understand that you have to justify the smashproof boxes over the polystyrene ones, because it's those that you sell, but it's wrong to give the impression that anything except your boxes will automatically lead to breakages.
The smashproof boxes used to be the best of both worlds when those 2-way ones were available (you could ship two 10oz mugs in the same box, thus mitigating the weight a little). Pity they aren't still available.