Looking For Shopify Developers

I have this app which creates a unique astrology chart based on user inputs. The user then selects a portrait option to print it and adds to cart. The challenge I am having is adding the portrait to cart and passing through parameters needed to fulfill the order via a 3rd party company Each order should have some unique identifiers so that I know which chart .png I should tell the 3rd party company to use to print on the Portrait.

Order has to go through Shopify first (once webhook returns payment went through), need to call 3rd party company API and point it to the .png of the chart.

I can share all necessary APIs and more information. I am looking for some integration help.

Need to know: Webhook, Shopify add to cart, API