When I received the "invite" from Spotify for the €0,99 for a 3-month Premium account I used my invite and my bank account to upgrade the Spotify account from my daughter to Premium. After that I tried to use the same invite and bank account to upgrade my own free subscription to Premium. Unfortunately I tried to pay with the same bank account as I did for my daugters Premium. Now I seem to be blocked for upgrading my Free account to Premium using the invite my daughter got, even if I use my daughters bank account (didn't try, but that's what Spotify wants to tell me in their FB help).
So in the end helping my daughter left me in a **bleep** position. I feel ripped off, as I should be able to upgrade my free account to Premium using the €0,99 invite.