[marklittlewood]

On your example, the most obvious buttons are bright pink saying 'Continue Checkout' with the VIP Membership program in a separate set of boxes on the side. They are definitely not presented as part of the workflow of the checkout process for a consumer.

'With this purchase you are activating your VIP membership' is smaller and in black. Incredibly easily overlooked.

It is possible that the JustFab UI designers aren't very good and have made a terrible mistake but the overall look and feel of the rest of the site would suggest that they have put a LOT of thought into steering their prospective customers down a very clear route to purchase and know EXACTLY what they are trying to do here.