Customers
A customer record links every sale, invoice and payment to a person or business, so "who owes me money?" and "who is actually worth serving?" have answers.
Adding customers
Name and phone number is enough to start. Add email for invoice delivery, and an address where you need it on documents.
You do not need a record for every walk-in. Create one when you will deal with them again, invoice them, or extend credit.
What it gives you
A trading history. Everything they have bought, every invoice, every payment.
A balance. What they owe right now, across all unpaid invoices and credit book entries.
Faster selling. Pick them at the POS and the sale attaches automatically.
Real reporting. Best customers by revenue, who has gone quiet, who always pays late.
Credit customers
Customers who buy now and pay later need a credit limit. Set it deliberately — see Credit book.
The limit exists to be enforced at the moment of sale, which is the only moment it can be. Once goods have left, you are negotiating rather than deciding.
Getting paid
Chase from Aging, which groups what is owed by how overdue it is. Work oldest-first: debt gets harder to collect every week it ages, and after about ninety days recovery rates fall sharply.
Send invoices with payment links so paying is one tap on a phone. Friction is a real reason invoices go unpaid.
Keeping it clean
Merge duplicates. "J. Okello", "John Okello" and "Okello John" are one person owing you three amounts you cannot see together.
Deactivate rather than delete — deleting breaks the history on past sales.
Record why someone stopped buying when you know. It is the cheapest market research available.