How does mc make enough income to survive? I forgot how much they pay to borrow the books, or if they pay at all
Technically, aside from lending books he also sells them, but yeah - he definitely doesn't have enough customers to come close to funding the bookstore normally (though his costs are essentially just what he needs to live there - the infinite books come free).
This was said very early on (somewhere in first 10 chapters IIRC) but Cherry/Irie has been one of, if not the first customers and she has been financing the bookstore ever since.
However, outside of what was needed to run the bookstore (which tbh. is probably basically a blank check from a company that has a duopoly on most products and more of a monopoly on supernatural goods, but the MC just doesn't even try to use it for anything but essentials since that's what it was supposed to be for) he doesn't have much money, which has been brought up a couple of times already.
Later this will somewhat improve
when he opens a book cafe, which will actually have more paying customers.