authorize.net and SHA-2 certificate upgrades

I am trying to test online purchases on our (temporary) website using osCommerce and Authorize.net (AIM module, sandbox connection, and their test credit card numbers). I had finally gotten it working on March 30, and then on March 31 it stopped working and all I get back from Authorize/osCommerce is a message about a problem with the credit card. It seems Authorize.net starting some upgrades on March 31, and they have suggested that I contact you (the hosting company) and refer you to their announcement (http://community.developer.authorize.net/t5/The-Authorize-Net-Developer-Blog/Authorize-Net-Begins-Infrastructure-and-SHA-2-Certificate/ba-p/49615 ). In short, that announcement says:

To ensure your solution works with the infrastructure changes, please validate the following:
Your certificate store includes certificates for Root 2 - GeoTrust Global CA
Your security transport—the part that negotiates TLS—supports SHA-256
Your solution does not connect directly to Authorize.Net using an IP address
Your solution’s firewall is not set to whitelist Authorize.Net IP addresses for outbound connections
Security Certificate Upgrades
The security certificates used by Authorize.Net for our sandbox and production payment gateway are currently signed using Security Hash Algorithm 1 (SHA-1). Along with the above changes, we will also update our certificates so that they are signed using Security Hash Algorithm 2 (SHA-2).
---------
Is there anything on your end that needs to be adjusted? Or is it all on my end (somewhere inside osCommerce maybe?)?

Thanks