Until minimum mileage is reached, throw away the charge for the idle time (speed lower than).
After min mileage is reached, idle time charge will be added to a meter instantly as it happens.
Other options:
Add to meter instantly. Regardless reaching minimum mileage or not, charge for the idle time will be added to a meter immediately.
Take from min mileage. Exchange with the equivalent amount of the minimum mileage yet left in min fare.
Keep for later. Keep the idle time charge hidden until min mileage is reached. On reaching it add the "saved for later" charge. Causes meter to "jump" when min mileage is reached.