Customers are usually quite happy to comply if they see your clearly stated payment policy right from the beginning. Send a second notice. Many businesses now pay 90 to 120 days after the purchase. Slow pay is standard procedure in many competitive industries. Send a reminder notice to speed things up.
Email is often the fastest, easiest way to touch base with a slow-pay customer.
Make a quick phone call. Then wait for an answer. Don't say anything. Let a moment or two of silence work for you. This method works surprisingly well.