たとえば

To whom it may concern,

I'm writing to confirm my reservation of flight AB8552 of 7 December 2014.
Due to the malfunction of your website, a fare of three seats has charged on my credit card since presumably three seats may have been reserved.
I request you to cancel two of them and refund them.
In addition, I want to know my reservation number necessary for check-in.
Thank you in advance.

Sincerely yours,

XXXXX XXXXX
XXXXXXX@XXXXXXX.jp