Hi lmrntx,
After visiting the Payment Method page within your Auctiva account by mousing over the "My Account" tab and selecting the "My Payment Method" option, the process you use to update the automatic payment method on file with your account will depend on whether you would like to create a new Paypal billing agreement or use a credit card as your automatic payment method.
From there, if you would like to create a new Paypal billing agreement for use as your automatic payment method, just click the "Edit my Paypal Billing Agreement" button at the bottom of the page and proceed as directed.
On the other hand, if you would prefer to place a Visa or Mastercard on file instead, you will just need to click the "Edit my Credit Card Information" button on that page, enter the details of the credit card you would like to use and then click the "Save Settings" button.
Also, I noticed that you posted a new thread about the listing error you encountered and I replied to that thread with some additional information a moment ago.
-Mike
[edited to bold text based on user request]