That is a good example.
Were you not able to cancel the plan you didn't want and sign up with the plan you did?
If that wasn't an option (ie, you wanted the better plan right away), quickest might be to request a refund of the bad plan and then sign up with the right plan.
If you request a refund the same day of purchase I've not had a problem with that (but it hasn't happened often). I had a somewhat similar situation, and just noted that it wasn't clear that X was not included, had no problem getting a refund. I then signed up as desired.
That said, devs should have an upgrade option that charges difference ?