If you compare this letter to the first collection letter, you will note there is a definite change of tone. You have eluded to the fact that continued lack of payment could damage the relationship you have created with this customer. This letter has turned up the heat a little bit but still remains polite and lets the customer know that you are ready and willing to help them in any way that you can, as long as payment is attached.
If, after this letter is sent you continue to hear nothing from your customer and subsequent attempts to call or email them fail, you will want to send out a third and final collection letter. The third collection letter raises the heat even more and brings attorneys and/or collection agencies into the conversation.