Hi, I am not sure purpose of this letter, if IELTS you need to start with Sir / Madam. Rest of the paragraph as Kevin said.
Dear Sir / Madam
I would be very grateful for any helps you are able to provide.
- Ask Strongly and say what you will do if it is not replace
I would be very grateful if you could replace the notebook in one week, if not I insist on getting a refund of my full payment.
is a casual way of finishing, formal letter you have to finish with - Yours sincerely,