Settings in Shopify
To begin this setup, you'll need some code from Shopify. Here's their doc on getting that code: https://help.shopify.com/en/manual/sell-online/buy-button/create-buy-button
Adding to Unstack
Once you've gotten that code, go ahead and add a custom code section to the page you'd like to embed on and paste the Shopify code there.
Help with installation
Please contact Shopify support for any code edits or advancements to this installation.