When a customer does not enter the correct credit card details, there is a warning sign added next to the credit card as well as the field's underline becomes red. This is fine but when customers are on smaller displays, they need to know to scroll back up the form to find the error.
The suggestion here is to add an error message just above the "Pay" and "Cancel" buttons (just like some of the system errors like the "declined" messages) which describes to the customer what the problem is so they know to scroll back up to fix the issue.
We are seeing significant problems with our customers and they wind up canceling the transaction!
