Thank you for your (month) payment. There seems to have been a mistake with your billing because you paid $(amount) too much. A refund check has been issued and you should receive the check within (number) days.
Thank you for your business. It is a pleasure doing business with you.
Sincerely,
(your name)
Другой вариант:
Dear (recipient's name),
We've made an error. Fortunately, our mistake is your gain. Your payment on your (company) account is only $(amount) and not the $(amount) you were quoted. We have corrected the error and will expect your payment in the next few days.
Sincerely,
(your name)
Другой вариант:
Dear (recipient's name),
It has recently come to our attention that you purchased a (item) for $(amount). We apologize, but that item should only be $(amount). Unfortunately, our sales clerk didn't realize this error at the time of your purchase. We are issuing a refund check in the amount of $(amount). You should receive the check shortly.
Thank you for your business. We appreciate your patronage.
Sincerely,
(your name)
Другой вариант:
Thank you for your advance payment of $(amount). We appreciate your payment. Since the time of your payment, we have made a few price adjustments. Our rearrangement works in your favor. We've recalculated your payment and we owe you $(amount). Your next payment will only be $(amount) and will remain at this lower rate for the remainder of our contract.
Sincerely,
(your name)