On Customers, set each credit limit and their balance, 90-plus overdue amount and a status flag fill in on their own. On Invoices, log each credit sale with its due date and any amount paid, and the outstanding, days overdue and ageing bucket work themselves out. On Payments, keep your receipt history as money comes in.
The Ageing Analysis splits every balance by how overdue it is, and anyone over their limit or with 90-plus overdue is flagged red so you act before a debt goes bad. Yellow cells are your only inputs. The premium version adds an ageing dashboard with bars and a printable A4 statement of account per customer. Opens in Excel, WPS or Google Sheets.