The best way to be sure is to get one in and try it out for yourself. Most suppliers will sell small quantities for you to try out first while making up your mind.
Some time ago, I needed some boxes for the heavy El Grande (15oz) mugs and 8" plates. Everyone and their dog sells regular mug boxes, but no one sells boxes for larger mugs or other items.
I had some doubled-walled boxes made up (not cheap, but you accomodate the cost in the end price) and, when they arrived, we tossed them around the garden - using the plate like a frisbee and tossing the El Grande as high as we could to let it come crashing down. We were so harsh on them that the boxes couldn't be reused afterwards (because we'd crushed the corners in) - but the items all still survived, even without any additional packaging.
I have some of the original smashproof boxes here (like the photo in
http://www.subli.co.uk/shop/dye-sublima ... d_719.html with the white inside, not like the video that has the brown inside). They were as rigid and strong as can be, but they were heavy. I understand the current type are not so heavy, but the video suggests they're still pretty strong anyway.
I have some of the BMS brown boxes. They are sturdy, but they are thin. They're ideal for individual packing of mugs if you're going to put them in a larger outer box for shipping, but I wouldn't want to use them on their own for posting.
Smashproof boxes have the advantage of taking up less space on your shelf (they're flat-packed) and are fine for one-offs but I tend to favour Poly Boxes wherever possible due to weight issues (2 mugs go for the price of one in poly boxes because they're under 750g, 3 mugs go 2nd class because they're under 1Kg, 6 mugs can go under 2Kg for special delivery, etc).
It's worth remembering that the smashproof box guarantee only applies if you also buy your mugs from subli. If the mug does break in shipping, the guarantee is only worth a credit for the cost of the blank mug against your next order of mugs from subli. You don't get back your production costs, nor the cost of posting the mug, nor the cost of the box itself - put all that together and that's a lot more than just the cost of the blank mug that you won't be getting back. And, if you've used someone else's mugs, the guarantee is void anyway.
In short, try out the different boxes for yourself - don't rely on guarantees. You'll probably find you're better off buying your boxes from whoever you buy your mugs from, as it'll save in postage costs.
have to order again more p&p nooooo lol