We do not have a specific integration between OptimizePress and Shopify currently as their system has it's own templating and site creation platform built in.
We have had some enquiries regard using OptimizePress based marketing sites or sales pages to sell products which are sold and delivered within the Shopify system - and we found an article which can advise on this:
You can create order links for products inside Shopify that can be linked from external sites such as sales pages that you create with a platform like OptimizePress.
You can find that article here:
This means you could create a sales page or marketing site with OptimizePress in Wordpress and link to a Shopify based checkout process if you wished, however we do not have any integration to add buyers from Shopify back into an OptimizePress based site at present - but if you were delivering content within Shopify this may work for you.