Dear …
Please find enclosed our invoice in the amount of __($amount)__. This billing represents the __(type of service performed/type of products purchased)__ on __(date)__. We want to apologize for the delay in getting this invoice issued. Unfortunately, we experienced __(briefly describe reason for delay)__, which caused an unexpected delay in updating our accounting records.
As (this service was performed/these products were purchased)__ __(# weeks/days)__ ago, the amount indicated on this invoice is due now. We ask you to remit this balance as quickly as possible to bring your account with us current. If you have any questions, please don’t hesitate to contact us.
It has been our pleasure to be of service to you and we thank you for your valued business.
Regards,