ASP Page - simple code change / error catch

I have a checkout ASP page - part of an ASP ecommerce site. It is a little like ebay - the buyer can buy products from more than one seller in a transaction In checkout, the buyer selects a delivery address for the products they have put in their basket. They can choose a different delivery address for products form each seller. So if they have ordered products from 3 sellers, they can have each order from each seller shipped to a different address. I need: 1. If all addresses required are not selected then cannot proceed to payment - needs a message please select address for seller2... 2. The addresses are selected by a dropdown and it needs to be clear which address has been selected by the buyer - this does not happen as present. A simple code exercise for a good ASP programmer. Need doing urgently please. Will take someone around 20 minutes I would guess.