Are you willing to send the giftcode to a trusted member? That way you can prove everybody wrong about the alternate method.
I'm not comfortable with that. And I can't give a guarantee to something that I don't comfortable with.
Anyway, I can do better for Trusted Member. I'll go first, and after you see the upgrades, and verify it is from gift card, then you will pay.
If you guys still don't exactly know where to verify whether your account is upgraded via promotions/creditcard/giftcard, you can see it in reciept page in your spotify account page.
Here's the screenshot:
http://imgur.com/muEkwR9Reciepts don't lie