To give another deviant a direct link to this FAQ answer, use the following code in your comment or forum post:
I have made an error while purchasing a Premium Membership, can I get a refund?
If you have made an error regarding the recipient of the premium membership, or a duplicate order, please contact the Help Desk within 24 hours, where we will be happy to help you.
Requests made after this time frame will be denied. Please note that all premium membership orders are final, and orders cannot be amended from 1 year to 3 months, or any other change of subscription period.
Please be advised that there is no need to make duplicate requests for a refund, should you not receive an immediate response.
You will not receive a refund after using some Premium Membership features such as username changes or Premium Member discounts.