delivery time

Hi,
I can choose the preparation time, (for example 20 min).
For pickup it is perfect but for delivery there is a lack of time between the store and the customer.
Example:
if a client create an order, he can pick it up after 20 min , but if he want the delivery I need to be able to add up the preparation time (20min) and the average delivery time (7min).
Is there any way to do this?
thanks!