It doesn't matter how strong the box is if you don't put enough bubble wrap around the cart. They could be tossing the box from the fourth floor to a pile on the ground for all I know. XD
It's true, no box will make up for shoddy packing. Though, it doesn't help when the box come back as flat as a pancake. The USPS boxes cannot hold any type of weight so if your cart is in a flat rate box at the bottom of the stack/pallet, good luck.
We had a ton of returns because due to toys being damaged during shipping when using the priority boxes. The boxes would come back completely crushed or ripped open. Now we just buy our supplies from Uline. It costs a little more initially, but in the end we have fewer returns.
This hobby is tough, people will drop XXX $ on a PCB or cart and then want to skimp out when it comes to paying for shipping. I just don't get it, that extra $5 or 10 will really make a difference.