Here I did a very rough integration of the Shopify Js SDK with ProcessWire, so that I could add more specific things to each product, when I talked to the client it was more important to have the potential to have more flexibility when uploading the product's content so we went this way of using Shopify inside PW.
My first mistake was rolling my own Js integration when I could have just used the Buy Button SDK (instead I used the Js SDK with Vue).
Second mistake. One thing I didn't anticipate that turned into a mess: discounts. I had to setup fields in Pw where I had to set the discounts (after setting them up on shopify), so a lot of double data entry on some cases.
I've done also things the other way around, setting up ProcessWire as an application that launches within Shopify, so that I could use PW pages as extra content management for Shopify product pages. (very complex product descriptions) unfortunately the project I did this for never launched.
Agree with @Pixrael the Shopify API is really easy to work with.