Dear Felix
It has been our pleasure to do business with you. It is with your good credit history that we have provided you with credit terms on your purchase. Payment within the 30-day payment period enables us to continue to provide our [products / services] to you and others at reasonable and competitive prices. We wish to continue to service you on these mutually-amicable terms.
A payment of $[AMOUNT] will protect your good credit rating and will enable us to update your account so that we may have the pleasure of future business with you. Please contact me if we can help you with any concerns you may have.
Regards
Lydia