Thanks for your post. Personally I wouldn't record a customer payment until I have the money as there's no way to track what is owing and if you report your BAS on a Cash basis you could be reporting amounts in the wrong quarters.
By allowing your customers to pay after delivery of goods you are giving them a trading account. You would need to create a customer card with the Terms set to the time you have given them to pay, say 14 days. Then create a sale for their goods and when you get paid record that payment through Receive Payments. This way you can keep track of outstanding amounts.