“three days overdue”
Overdue means past a return or payment deadline. Three days overdue means the book was returned three days late, leading to a discussion of a late fee.
A2-37 / CEFR A2 / EPISODE 37
Paying a library overdue fee
SceneAt the library counter. Frank has come to pay an overdue fee.

WORDS & CONTEXT
Overdue means past a return or payment deadline. Three days overdue means the book was returned three days late, leading to a discussion of a late fee.
You're all set. means everything needed is done. The librarian has taken the three dollars and confirms the payment is complete. It happens. reassures someone: "These things happen." By saying no further apology or steps are needed, she creates a contrast with Frank, who takes his mistake very seriously.
Twice means two times, and a due date is a deadline; here, the date the book had to be returned. Frank explains his first late return: he was rereading the book and forgot the due date. Here read is past tense, so it is pronounced like "red."
Here a reminder is something kept to help someone remember, rather than a notification. Frank keeps the receipt as a warning to himself not to return books late again, showing his meticulous nature.
A folder here is a file for keeping paper documents such as receipts. Frank plans to put this receipt in a folder labeled "Mistakes" so he will not repeat the error. The punchline is his thoroughness: for the librarian the matter ended with the payment, but Frank even keeps a record of it.