Pallet Corner Protectors
Without them the strap concentrates all its force on the top carton edge, which either crushes the box or forces you to under-tension the whole load. A protector spreads that along the edge so the strapping can actually do its job. Cardboard angle is cheap and single use; plastic and moulded versions survive a return loop. Length should cover the stack. See pallet wrap, pallet cartons and shrink wrap.