It's broken right now apparently as part of their "new look" oops... You can find the link if you select "account settings" under your profile name in the top right corner. On the first page at the bottom you'll find "shipping" below "products & services". That'll bring up the shipping page with "shipping calculations" is at the top with "update" link on the right. However when clicking that update link I get the awesome message: "Sorry — your last action could not be completed"
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