There are a few reasons why you might have been charged - please see below. See your billing history by logging in to your account.
One Free Trial Per Customer
Free trials are limited to one per customer, so signing up for an additional free trial may create a fresh account which bills immediately.
Credit Card Authorization Hold
Napster may check the validity of your credit card by using an authorization hold, which is a temporary dollar amount which looks like a charge (but isn't). Your credit card company controls the amount of time it takes to release the hold. The hold may temporarily reduce the amount of available credit on your card.
More Than One Account
Napster charges a monthly fee per subscription. Sometimes, users have signed up for accounts with different email addresses. If you see more than one monthly charge on your credit card, you may have more than one Napster subscription. See this article for steps on canceling Napster.
End of the Free Trial/AUTO-RENEWAL
When you signed up for the free trial, we might have asked you for a credit card number. If you didn't cancel before the end of that free trial, we automatically bill you for the subscription fee after the trial ends. Free trials that require a credit card automatically convert to paying accounts unless canceled before the trial period ends.
Change of Subscription Type
A temporary line item may appear on your credit card if you change from one subscription level to another. However, you have not been charged twice. If you move to a lower priced subscription, you will receive a prorated credit. If you move to a higher priced subscription, you will be charged the difference.