Delivery fees should be calculated from the restaurant's location by distance (simple using Google Distance Matrix API).
For example:
Restaurant is 5km away from the customer.
Minimum delivery fee is 3€ and includes 2km. Delivery over 2km is an additional 0.50€/km.
This would mean 5km from the restaurant: 3€ (base fee) + 1.50€ (3km x 0.50€) = 4.50€ delivery fee.
This would also mean no need for drawing any circles or polygons for delivery zones, just a simple matrix which can be done with Google Maps and Google Distance Matrix API.