The Skiller can decide whether he wants to give up to 5 minutes free. Its good in order to check if they match. And in case that the 5 minutes free was enough for the whole service the user can tip the Skiller and everyone will be happy

How do you prevent people from abusing this 5 minute grace period?
We don't. its up to the Skiller to give them. If the Skiller notice that the user is not serious he can end the call immediately.