Find Max # in zip code range and then sum all numbers in that zip code

In a nutshell I have a large spreadsheet with several zipcodes and the corresponding towns for those zip codes. I need to write a formula that says to look at the zip codes and tell me if the city customer count is the highest, if it is then sum all the city customers into that one city.

I know that sounds weird, but I'm working with MapPoint and it will not let me load a zip code more than once, so for simplicity sake, I'm loading all the zip code customers into one city.

I've attached in tiny sample of what I'm trying to do.