When You try to check out with real time fedex the zip code is not being sent to FedEx to calculate the shipping fee. This happens on anonymous checkout on the Deliver to block.
I also noticed there are arrows for dropdown box on a couple fields. see pics.
CS cart basically said that in the "deliver to block" in check out that you can not change the shipping address there and get FedEx live quotes because you cannot change the zip code in this block. FedEx needs the zip to make shipping quotes.
This is confusing for the customer. The deliver to block should not be made to edit if it serves no purpose.