Custom Shipping Calculator for eBay listing

We require a postage calculator for our eBay listings where customers can simply input their postcode and a shipping amount is displayed. We need something that our customers can input their postcode and will recognise what zone the postcode is, and then be able to retrieve the price for that zone (We have 18 zones in total but once again would be good if we get something we can alter should we need to add, subtract or alter zones - not 100% essential but would be a plus). We need to be able to edit the postage amount that it will spit out for the zones as we sell many different products all costing different amounts to send.