You might have not been eligible for the offer. This explains getting charged the standard rate. You might not be eligible for a number of reasons such as:
You've purchased a similar offer before
You're using a payment method that has been used to pay for a similar offer
You've already been Premium in the past
Your payment method didn't meet the requirements for the offer (you had to had signed up with a recurring payment method, such as a credit card)
If that doesn't seem right, reach out to our team viaTwitterorFacebook. They'll be able to take a look at your account to see what's going on. They'll also be able to see what the possibility of a refund is.