> you should check to see if your library has online services - mine lets you renew online, even if it is overdue (unless there's a waiting list).
I can't resist mentioning my library's bizarre set-up here: if books are due on day X, but are in the drop-box at the time of opening on the next open day (for that branch, so one can game the system a bit by returning to branches that are closed on particular days), then they are counted as having been returned on time.
Once, I took advantage of this, but, just to be on the safe side, also renewed the book online. (I've had problems, usually eventually resolved, with re-shelvers forgetting to mark the books as checked in, so that fines accumulate for a day or two.) It was treated as overdue. When I asked about this, they told me that it was the intended functionality of the system.