Offer to pick up payment in person. Use a good location system. You may occasionally have a customer who racks up a big bill, then disappears. The customer may have moved without letting you know, gone out of business, or changed their email or phone. They can find your customer quickly at low cost.
Most people are honest and eager to stay on top of their debts.
Usually, a customer fails to pay you because they forgot, lack organisation, or have an unresolved issue still pending.