If your site is giving someone the following error during a transaction on soapbox:
“Payment Error: (15005) - Processor Decline - This transaction cannot be processed.”
This is an indication that the transaction has been declined by the bank that issued their card. You may want to recommend that they reach out to their back to ask why the transaction wasn't approved.
Note that there isn't anything that your credit card processor, nor we at Soapbox would be able to do to resolve this issue.