What is the URL that Flexify is giving you? You should be able to paste past that into Facebook Catalog and upload your feed. When you go to the Apps section in Shopify and click on Flexify, that URL at the top is the one you should be using.
We help ecommerce brands create, manage and scale profitable PPC campaigns across North America, Europe and Australia. We also help with Conversion Rate Optimization: TakeSomeRisk.com