Tapas Adhikary, thanks for the fast reply! I'm loading the products directly from Gatsby graphql by using the gatsby-source-contentful plugin. So if I understood correctly, I will have to use Apollo Client inside the checkout function to query Contentful and get the inventory?

Great guide man, thanks for writing it! It works great when you manually add the products through products.json, but I am currently trying to use this is on a website where I am getting the product data from Contentful / graphql. How would I go about adding the products from the graphql query to products.json or the checkout function? Thanks again and sorry if this is a stupid question but I'm fairly new to web development.