This can manifest in two ways: 1) the storage space physically necessary to hold the TP and 2) the capital tied up in owning this before you need it.
To put it another way, maybe poorer people can afford to buy in bulk, but in their apartments they have no place to put a gigantic package of toilet paper. Or, maybe they choose not to tie up $25 in securing future TP when they need that liquidity to ensure they can make a car payment this month.
In either way, they get something of value by not holding inventory in their house. That's why it costs more. It's not completely clear that they'd be better off if they could lock in low TP prices, even if they did have the cash on hand.