Hi Rahul,
No, the sender details (i.e. the Controller of Examination Pune University) is already printed on the sealed envelope that you receive from the university. There is no need to write any other details on the envelope. Just stick a print out stating the address of recipient (WES, in our case), and hand it over to the post office guy. He knows the procedure for posting ECA docs, so that won't be an issue.
I recently received acceptance mail from WES for my PU degree, so I can confirm this procedure is full-proof!
Hope this helps.