Dear Mrs. Black
My name is XYZ. Recently I was renting flat in your apartment/. I am writing because I left my watch, wallet and the book. I think there are in the night-table. I will be grateful if you send me back all this things. I pay for a postage. I am so sorry for trouble, I hope you will be understanding.
I look forward to hearing from you soon
Yours sincerely
XYZ