So I tried again from the mail after I had logged into my Microsoft account. It had a redeem now button. I hit that to add it to my cart, applied the promo code, which made the amount due 0.00, went to check out, got this:
We're sorry but Microsoft Display Dock is currently unavailable. Please return to the Shopping Cart and remove the product to complete your order. We apologize for any inconvenience.
We're sorry but Microsoft Display Dock is currently unavailable. Please return to the Shopping Cart and remove the product to complete your order. We apologize for any inconvenience.