My view is shaped by the observation that libraries in my neck of the woods in fact attach an account balance to your card. You can pay your balance self-serve using a terminal that accepts bank cards.
We live in the same neck of the woods, based on your HN profile; there is a balance tracked at Vancouver Public Libraries that is based on late fees. It's also worth noting that VPL wants to get rid of late fees as well, based on this article from a few months ago. https://bc.ctvnews.ca/vancouver-libraries-offering-fine-forg...