Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
If you updated the banking details, then you will have to de-register the Kindle and re-register it. This will update the account with new card info. Go to Settings> device and/or Registration and de-register from there. Then re-register it from the same menu. That should fix it.
If you have any difficulty on following the steps above, please let me know and I will continue to help you