Sign up for Shopify Buy Button account.
Add your products to the account.
Place embed widgets on your Webflow site. One for every product and one for the Cart.
Create your Shopify buttons and paste the generated code into your Webflow embed widget.
Copy and past the Shopify Cart code into the embed widget.
You are all set. When you update your products through Shopify your Webflow site will update aswell.