I'm creating an app that allows users to create their own design for printing. I need to automate the whole process from sending the result design file to Printful to checking out the delivery and payment data to Shopify but I totally cant understand how to implement it. Looks like I need to add (update?) product variant via Printful API and then to handle updated variant via Shopify API to get checkout page url somehow. Modifying existed variant doesn't look correct so I probably need to create new variant first but Prinful doesn't support creating new variant for products synced with e-commerce platforms.