you would need to create a database table that contained all off the post codes within your 20 mile radius.
add a column that will mark the post codes between 15 and 20 miles.
The user will enter the shipping post code on the checkout page which is then sent to the confirm page.
On the confirm page, create a recordset that will be used to lookup the post code to see if it is in your database.
use the set session value behavior to set a session that contains the column making if the entered zip is between 15 and 20 miles.
Use that session in your shipping rules