Apparently we're not reading the same comment. In this example, the rich person pays cash for the 20-pack and probably earns interest on the savings. They have no need to pay interest on a loan. The poor person pays cash for the 2-pack and then pays again next month, and maybe takes out a payday loan to afford the rent after they have to call in sick, and there's your high APR. This is not a benefit. What am I misunderstanding?
The entire premise, where I suggested this was an easy mutually beneficial profit opportunity, for the rich person to lend to the poor person to make the bulk purchases? I'm not sure you're keeping up with the exchange here.