An Apology for Not Crediting Payment can go a long way with your customer.
Books not up to date? Customer complaints coming in that they have indeed made their payment? Tragic!
Maybe there was a delay in your accounting department, and if that's the case you will want to clear up any discrepancies quick, fast and in a hurry...
as an apology for the inconvenience not crediting a customers account is the first step you must take to ensure further positive customer service.
Thank you for your assistance in helping us to locate your payment of [date] which had not been credited to your account.
Please accept our apology for this error and for the overdue notices we sent to you.
We appreciate your patronage and are sorry for the inconvenience caused by this error.