After reading your letter of [date] , I can certainly understand why you are frustrated.
Please accept my apology for the delay in refunding your money for [basis of adjustment] Our bookkeeping department has been instructed to issue a check to you at once, which you should receive within a few days.
I am grateful that your letter was brought to my attention and I appreciate your perseverance in settling this matter. Once again, I am very sorry for the inconvenience this has caused you.