There are two reasons why you may see more than one charge.

If you received more than one confirmation email for your order:

Multiple confirmation emails indicate that you have completed more than one purchase. For information about cancelling please see the order email.

If you received only one email confirmation, or if you tried to order twice and one order attempt failed:

When an order is submitted online there are two processes that take place: First our system verifies that the payment method is valid and that funds are available for the amount of the order, and the funds are reserved for the vendor. This is called an authorization. Second, the actual charge for the purchase takes place when the order is successfully completed.

If you see an authorization on your credit or debit card for a failed purchase attempt, it will be released back into your account according to the issuing bank's policy. This usually happens within three business days.