For consumers you’re definitely right, but I don’t think this is true when buying by the truckload. Paper manufacturers produce both very high-density products (office paper) and very low-density products (toilet paper). Office paper is so dense that you have to ship it in trucks that are half-empty due to safety regulations on truck weight, so I’d expect a customer that needs both products can receive toilet paper in the same truck without much marginal increase in fuel consumption. I don’t work in the industry though, so I admit I’m mostly speculating.