Thanks for getting in touch with us, we'll gladly help you out.
Right now, the 30-day trial is only available to new users who haven't had a Premium subscription before. If you had the trial or a recent promotion, your account doesn't qualify for the trial, and the full price is charged.
Since you set up the subscription via Boku, texting it to cancel should've sucessfully stopped it. However, if you want to make sure you won't be charged for Spotify again, it'd be best to contact Boku.
All the best.
“Music acts like a magic key, to which the most tightly closed heart opens.” ― Maria Augusta von Trapp