Refunds on Bundles\n\n You can receive a full refund for any bundle purchased on the Steam Store, so long as none of the items in the bundle have been transferred, and if the combined usage time for all items in the bundle is less than two hours.

You have to log in with the account that bought the coins. If the purchase is very recent, it may not appear on the list, but if you have order details you can fill out the refund form as well.