Anytime you ship a large box like that without support on the inside of the box for the walls of the box it will collapse. It's a simple fact of physics and the way these parcels are handled by ALL shipping firms.
As someone who ships a few parcels now and then I have also found there is a HUGE difference between inexpensive packing tape and good packing tape. But even with that if you have nothing but air under a box flap it's very easy to separate it from the packing tape.
Postal regulations actually require 2" of packing material around the item. Take some cardboard and make a box that will hold a brick. Insert brick and mail. Box will arrive just fine. Take the same box and mail empty. Chances are the box will arrive smashed. You can do the same thing with UPS and get the same results. That's way many vendors learn to pack boxes TIGHT and use the correct packing material.
When I used to order a lot of domestic cigars they were packed in such a way that once you opened the box and removed the boxes of cigars you could not get them all back into the box with the packing material used. Kind of like digging a hole and then filling it with the dirt you removed. You always end up with extra.