We have added a new keyword in Case Email Template: ##CASE_CLIENT_CLOSE_URL##

You can use this keyword to request the Contact/Client for a case to close their ticket. For example

" ... If you think this resolves the issue, please click on the following link:
##CASE_CLIENT_CLOSE_URL## "

or

" ... <a href='##CASE_CLIENT_CLOSE_URL##'>Click here to close this case</a> "

Your contact will receive the link in the email, and if he/she clicks on the link, the case will be closed.

And of course if you have specified the profile "Email Notification When Portal Updated", your Agent will receive an email notification when the Contact/Client closes a ticket.