Writing a letter to a company

Hello,

I have a couple of questions about writing a letter to a company. This question has probably already been answered somewhere, but I have trouble finding it on here.

In this letter, I have to ask an air company to cancel one of our reservations. We accidentally did a double booking, which now leaves us with a one way trip from Brussels to Barcelona -and- a trip from Brussels to Barcelona and back again. If we can cancel the first trip, then that would mean the company has eight seats to sell again, so it's in their benefit. A refund is not necessary, but would of course always be nice if possible.

My question now is, how can I put this in a decent, formal letter? And how do I begin and end a letter when I don't have any person to direct it to?

As you can see, my English really isn't all that great and all help would be greatly appreciated.


Thanks in advance

Eenuh