If you are using Shop Manager Pro, follow instructions found here: http://helpdesk.watersidedesigns.net/support/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=12
If you are using Roman Cart, follow instructions found here: http://helpdesk.watersidedesigns.net/support/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=11