The librarian who waived your late fee was betting you’d come back. Prove her right.

Letting something go is sometimes an investment rather than leniency. Waiving a fee means choosing a reader over a rule, on the assumption that you will keep using the place, and that is a judgment somebody made about you with no evidence to go on. The money was not the point. Keeping you inside the building was.

Go back, return things on time, and use the library for something beyond what you were told to read. Ask for a recommendation, which is the part librarians most want to be asked for. If the fee was ever a real problem, ask about the fine policy rather than quietly staying away.