I setup custom engraving for products in 2006, 2007, and on. Earliest hacks on Shopify. Once Shopify introduced line item properties, it became rub-a-dub-dub three men in a tub easy. Just do that. It is a no brainer. Capture an engraving data set, assign it the line item you send to the cart, and you’re good to go. This pattern applies to capturing dimensions for items that have unit prices and lengths and widths, or initials on a monogram for a baby bottle, or just about any kind of collected data. Has zero to do with Storefront API, this is built in to Shopify. When you POST an item to cart, those captured data points are saved. You can do a lot with that pattern, including connecting different products. For example, selling a picture frame, with a custom matte, color, and size of border, and whether to use anti-reflective glass or not. The sky is the limit.
LINE ITEM PROPERTIES FTW… easiest and best update Shopify gave us way back.