Question: How do I get osCommerce working with SSL (you may receive an error about secure and non secure items shopping cart)

Answer: You'll need a dedicated IP address and SSL certificate installed on your site.  3Essentials sells RapidSSL certificates:

• details on RapidSSL certificates are available at: Information about Rapid SSL Certificates sold by 3essentials
• For current pricing and ordering information, please submit a request to our sales or support teams.

Once you have a certificate installed on your site and have installed osCommerce, you need to configure the Oscommerce for the secure connection, which it doesn't do by default.  You can refer to the following KB article from osCommerece for more details:

http://www.oscommerce.info/kb/osCommerce/General_Information/Common_Problems/75

Contact osCommerce for further support of this if needed.