Suppose you live in a location where you leave the materials out for return pick-up, but scavengers collect them rather than the shipper?

It's not feasible for the customer to stand guard over the packaging. It's effectively discriminatory to impose a financial penalty based on residence (a factor which cannot readily be controlled or changed).

Some alternative means of assuring returns would have to be arrived at.

Tracking, centralised or localised return depots, or other mechanisms, might be possiblities.