You may add a credit card using this guide 

STEP 1 : Login into our customer portal ( Link here )

STEP 2 : Navigate to the Add credit card button at the top of the navbar, or load this page ( Link here )

STEP 3 : Fill up the information form and click create account

STEP 4 : A notification will appear, and then you can select the card and Set it as the default payment account.

This allows you to pay new orders/invoices via this credit card by default.

To remove auto debiting of your credit card for new invoices, you can remove the Auto Debit feature via the Remove Auto Debit function.