Wrong calculated shipping price shown to not-logged-in buyers

I was notified by a buyer today that when he looked at my listing it showed the shipping would be $7. Then when he made the purchase it charged him $13.74. I have tracked this down to him not being logged in when he first looked at the item and the shipping calculator apparently used MY zip code to come up with the $7 price. I logged out of my account on the app and also on a PC and sure enough when I look at my calculated-shipping listings, they all show super low prices based on MY own zip code.

Is this a bug or am I doing something wrong with the listings?