If you have a Shopify store, you can add Facebook as a sales channel by using the Facebook Shopify app. In order to set this up, you must have a Facebook Commerce Account approved for your Facebook Business Page.
For help setting up your Facebook Commerce Account, please refer to this article.
Once your FB Commerce Account is ready, you can set up Facebook as a sales channel for your Shopify store.
For help setting up FB sales channel on Shopify, please refer to their Help Center.
Please note: For reasons unknown, Shopify does not automatically sync new products from your Shopify store to your Facebook page.
After you add Trendsi products to your Shopify store, you will need to add those products to the Facebook sales channel. There is also some lag time before you will see the products added to your FB Shop.