Idea for speeding up transactions

I have noticed that getting shipping information and then calculating shipping costs adds a couple of emails to the transaction. This can even add a couple of days if the buyer lives several time zones away.

But If on the member lookup someones zip code was included in that information you could calculate shipping costs right away and send it back with your first reply to the buyer.

Sometimes you can look at their current ads and get it from there but if they dont have an ad posted you cant use this.

What do you think of this idea?