People with more cash on hand save money by heading to Costco and purchasing enough bulk toilet paper to polish the tushies of a small country. For example, a $24 30-pack saves way more money per roll than a $5 four-pack. But poor people don't always have $24 lying around, so they often go for the less cost-effective (but cheaper overall) small packs.
Even worse, buying in such low quantities forces them to repurchase such items more frequently, meaning they don't have the luxury of stocking up and waiting around for a blowout toilet paper sale that could save them even more money. Not to mention how many don't have the money to pony up for a Costco membership in the first place. They're more likely shopping at retailers that don't offer such bulky discounts. And this isn't because poor people aren't aware of the cost difference. The same study found that at the beginning of each month, when paychecks are freshly cashed, the poor are just as good as anybody else at hitting sales and buying in bulk. It's at the end of the month, when money is stretched, that things get more desperate. And expensive. And scratchy.