>I want it to be as simple as possibe for the customer so that he has not to open port and all that..

Ultra VNC SC "single click" is your answer.
You make a special version of Ultra VNC Single Click.

It makes an exe specific to you with all the configuration built in, and even your own graphics.
It will reverse-connect to the address you specify (your office typically)

Your customer does not have to configure the app or open any ports.

Of course, your office will need ports open, and you will need to keep the listener application running.

Your customer can either double click on the exe, or you can make a menu item that launches the exe using ShellExecute()

You can easily google for the details.