I send packages for my mom's business through UPS. The packages are two or three sheets of corrugated cardboard taped together with paper stencils sandwiched between. I used to write "Do not bend" on them until someone told me that's just asking for it to get damaged which did happen. They'd be folded in half and the stencils shredded by the time they got where they were going. When I stopped putting warnings on them they stopped getting crushed and folded.