Had some trouble finding the Demo link until I looked over in the sidebar.
Nice concept. I think the progressive feel of the whole process and the adaptation to small screens/windows are particularly good in this one.
I can also imagine a development of this idea where you prompt for the user’s physical address on the sign-up form as well. Then you could use geolocation with an optional override of the user’s country to show pricing in the local currency and a localised range of payment options, too.