The Authorize.Net AIM payment option returns a "Credit card is invalid" message when NOT in test mode even though the credit card it valid.
The credit card invalid message will result if PHP on the server can't use SSL. The AIM module requires either Curl+SSL support in php or a command line Curl with SSL support. Ask our service department to run some tests on the server to confirm that there is a command line Curl available and to see if it can access any URLs with https. This seems to be a common problem with some versions of Curl.