Dear -----,
I am extremely grateful to you for sending back the bag containing important documents that I had left in the bus yesterday.I was very upset and pensive when I found the papers missing.I had one important meeting today and you saved me , because if it hadn't reached in time I could have lost my job.
I will always remember your favour and would be glad to help you in return someday in anyway I can.Thanks a lot!
Thanking you,Yours Gratefully,