it's good of you to step up and have this discussion. It seems that justfab is using a Conversion Rate Optimisation trick that you're either not aware of or that you're deliberately downplaying. Put simply, the UI is set to have VIP membership activation default to ON. Those people who are not paying attention will not read the information in the right hand side bar, will check out and will mistakenly purchase the membership.
The simple way to fix this is to have NO DEFAULT. When a user clicks check out, you should show them a page with both options equally balanced. For example, you could have two radio buttons with neither option preselected. That way the user has to make an explicit decision to choose to become a member or not.
This is your opportunity to put your money where your mouth is. Get the justfab team to redesign it. Make it impossible for users to opt in by mistake. Rely entirely on users making an explicit choice. After all, if you genuinely believe that all of your valued customers opt in to the VIP membership by choice, then this redesign will not have any effect on your revenue.
Ball's in your court. We're all interested to know how you will play this.