A pro forma invoice is a preliminary bill of sale sent to buyers before the shipment of goods or the delivery of services. It is used to declare the value of goods for customs and to facilitate payment.
Based on this, a pro forma invoice is not required when dealing regularly with a customer. In such cases, the buyer and seller have already established a relationship, and the buyer is familiar with the seller's products and pricing. Therefore, a formal pro forma invoice may not be necessary for the transaction.