Just go to your local shop and buy the version you want, you won't save any money by purchasing the key only even if you could buy it where you are. You are paying for the licence, regardless if you download it or buy it in a pc shop.
I bought a disk version of windows 7 home premium and have installed it on two PCs, thinking when the trial period ran out on the second one, I would just follow the activation links and buy the license online. But now for some reason I get to the microsoft site where I can only buy with a US billing address for my credit card, which I don't have, being in the Netherlands Antilles. I want to buy the application code, how can I proceed? Windows has already started running with reduced functionality on this PC.

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