The following phrases suggest natural ways of ending many types of routine letters.
May we hear from you soon?
Willyou please give this matter your (prompt, urgent) attention.
We shall appreciate your writing and telling us your wishes.
Please send us your answer as soon as you can.
I am looking forward to hearing from you soon.
Will you please let us know what action you want us to take.
Your promptness in replying will be much appreciated.
I am looking forward to your reply.
We appreciate your interest.
It is a pleasure to be of help.
Thank you for giving us the opportunity to be of service.
I hope this information will be helpful to you.
We look forward to the opportunity of serving you again.
Thank you again for writing.
If we can be of any (further) help, be sure to let us know.
It was nice to hear from you. Please call on us again.
Thank you for your courtesy.
You may be sure that we appreciate your efforts on our behalf.
We appreciate your writing to us.
We are sorry for the inconvenience caused you.
Thanks so much for your pacience.