Are you sure that you need to buy any certificate to distribute through the Microsoft Store? I quick search seems to indicate that you can use a self-signed certificate to submit your app to Microsoft, and then Microsoft will sign it themselves after they've approved your app.
Even if a certificate is required, I'm a bit surprised if EV is required, and not the less expensive OV instead. I've been distributing Windows software outside of the Microsoft Store for several years, and Windows has been perfectly happy with my OV certificate the whole time.