How to tag your products
This article guides you through how to assign your own Shopify tags. Note this option is not available for the Starter plan.
In your Shopify admin, go to the product you want to sell. And add a unique tag for that product:
In the case above, the product is tagged "apron". Then all your customers have to do is comment "sold apron" to purchase the product on ANY post made by your FB page.
You'll want to make sure the tag you assign is unique to that product.
The way to check this is to go to the PRODUCTS tab within the app and verify that it shows up. That tab shows all the products in your Shopify catalog that have unique tags. Any tag within the PRODUCTS tab can be sold through comments.