As is any generate case, a buyer responsibility is to settle the payment promptly and a seller responsibility is to provide the goods or service promised to the customer in a way that is agreed by both parties. By making customer having to keep calling without any reply is pure poor service. Busy should really not been a good excuse because you choose to sell and engage in this business. So please, any seller here, if u are unable to provide the service as you promised, propose an alternative solution promptly, the buyer in general just want to get their stuff and not looking for trouble.