Hi Pipat,
Thanks for contacting us. My name is Lovely, your iTunes Store Advisor.
I understand that you are requesting a refund for an item that was purchased accidentally. I would want to look into any options available to me as well if I unintentionally purchased something, and I will be more than happy to assist you today.
Pipat, I reviewed your account and found that there is an unpaid order on it. Unfortunately, I am unable to make certain modifications to your purchase as the unpaid order would prevent our system to do so.
Since your credit card card has been declined by your financial institution, please contact them to find out why the card was rejected. If they're able to resolve the issue, you should be able to pay for the order using the same card and then continue purchasing with your account. If they can't resolve the issue, you can pay for this order with a different card or with store credit.
To review what the purchase was for and how to pay for the order, please see the steps in this article: