I don’t understand what you’re asking. Are you trying to use headless Shopify with WordPress?
what should happen next, after someone hits "buy now" on your website?
What is the main Shopify-feature, that makes you not want to go another route altogether?
I would suggest one of the following options:
if you really „only“ need to sell through an existing WordPress site: habe you had a look at WooCommerce? I know this is Shopify turf and I’m not trying to drive away clients, but really, approaching this from your merchant view – if your requirements are a WordPress integrated ecommerce solution: that already exists. Maybe even a simple „buy now“ button from PayPal (or any other similar service) would be enough for your use case?
Integrating it can be done with rather moderate resources. Building a headless Shopify integration would be a whole other magnitude of project.
Either way - we have experience with all of these solutions, just in case if you want some help or advice.