Dear (recipient's name),
Enclosed is an invoice for your most recent purchase. We're so happy you chose to do business with (company) and hope you'll be happy with your purchase.
Just a quick reminder to let you know we offer an incredible discount for early payment of all invoices. If we receive your payment in full within (number) days of purchase, you will receive a (amount)% discount. If you choose to pay within (number) days, we'll offer you a (amount)% discount. We hope you'll take advantage of this money saving opportunity.
We appreciate your business and hope to see you again soon. If you have any questions or problems with your shipment, please don't hesitate to give us a call.
Sincerely,
(your name)