There are a few options:-
1. Get down to Immigration taking your evidence of the flight ticket/booking information and explain to them the situation.
2. Send them an email, explain the situation. Still need to go to Immigration Tower to get your extension though.
3. Call them. Still need to go to Immigration Tower to get your extension though.
4. Do nothing and go to airport. Possible repercussions.
5. Do to Macau/Shenzhen but will be more expensive and you have already ruled it out.
Just get your ass down to Immigration and fix it!