Dear Sir, 
Dear Madam, 
I would like to register FinalWord.
Thank you for your letter.
Further to your letter of
In reply to your letter of
Further to our telephone conversation,
It was a real pleasure to welcome you as a customer
If you require any further information, please do contact me.
Call me personally if I can be of any help to you.
Best regards,
Best wishes,
Yours sincerely,
Yours faithfully,
Yours truly,