We disabled the Credit Cards Payment Method on all invoices recently while troubleshooting some issues with our Payment Gateway.
If you tried to pay an invoice recently you may have noted that Payment Method wasn't available.
It is now back online. Sorry for the inconvenience.
An Unplanned System Outage occured today Sept. 9, 2015 from around 10:00am - 11:39am
It has been resolved. All systems back to normal.