Post code calculator- any help would be hugley appreciated

Hi

we send parcels nationwide and i am trying to get a spreadsheet set up which will automatically calculate the costings.

Ideally, i want anyone to be able to type in the post code and weight of an item, and it will calculate the cost.

i have attached an example spreadsheet. costings will be based off the main formula + prefix of post codes.

for example;IF(B3<20,13.29,(B3-20)*0.27+13.29)*1.088 - EG; When i type in post code in A3 as AB11 2AF- I want it to add on £22.50 to IF(B3<20,13.29,(B3-20)*0.27+13.29)*1.088

any help would be great