Get it from karachi, just before entering motorway. They are open 24x7, with small breaks ( I reached 6.45 am they said will open at 7 we are closing cash).
Charges are rs. 200 and balance on top.
You need car registration book or card only.
It is useful for karachi and Hyderabad, then usefull at lahore/Islamabad motorway.
In between and on M5 (sukkher multan you need to pay cash every where).
You can check all details and expense report on a mobile app also notifies you when ever you car is charged on a mtag.