Dear

Enclosed is our check in the amount of $ which
represents a refund for your inadvertent duplicate
remittance for payment of (purpose)

We are pleased that our bookkeeping department discovered
this overpayment so quickly.

Thank you.

Dear

Having received the merchandise you returned to us on
(date) we are enclosing our check to you in the
amount of $ (amount)

Thank you for taking such care in the packing of the
returned merchandise.

We are sorry that circumstances prompted the return
of this merchandise, but hope that you will continue
to allow us to serve you in the future.