> Costs users $3/week. Which is non-trivial, but less than a payday loan company.

That does depend how much and how often you need to borrow money. If twice a year I need to borrow $50 but pay it back the next month, that's equivalent to a 13000% APR loan rate (~$150 for the convenience of borrowing $50 for a month twice is equivalent to borrowing $50 and paying back $125 the next month, 150% per month or 13000% per year).

It doesn't have the same risk of spiralling out of control, but for small short term loans this is a really expensive thing to do.

Edit - For comparison, $150/year is like borrowing $2800 at 10% and paying it off in a year.