I'm also running into the same problem. I clicked the link in the email. I pick the mini color. I check out but still see the $49 + tax amount. I checked and I am registered. In the URL it even has the discount code.
Quick update since I am having a similar issue with my order. I did connect with Spotify support and spend probably an hour with them. The support dude was very nice:) and helpful but they did not get back to me.
Since I am a software engineer here is my 5 cents on this issue. This may help people around to understand the issue with canceled orders.
1. Spotify has a single google promotion code locked for every user.
The support chat really is a waste of time, they offered to escalate to a special team which never responds in the time frame they set, this is on multiple occasions. I am very familiar with customer support industry and messaging support as well and their team frankly is a disappointment and the main reason I am no longer a Spotify customer.
I've gone back and forth with support at least 5 times on this issue. They are prompt in responding to me but clealy have no clue as to how to resolve the issue. I did just forward them the post here about the promo code having already been registered and that causing Google's backend to mark the promo as having already been used.