Can't say I've ever had a problem. In the photo the two pizzas should have been stacked on top of each other so the boxes would help support each other. And the straps should have been placed in the direction along the length of the rack - not pulling down on the sides of the boxes.

But if you do pick up pizzas often and have a decent length ride then spending the $15 or so for a good insulated pizza delivery bag would probably be worthwhile.