Our Product Accessories app works by duplicating the add to cart button, so that when our button is clicked, it fires a function to send the selected Accessories, and then simulates a click on the original button to send the main product to the cart like normal.
This unfortunately means that if a customer uses a Buy Now button to reach the checkout page, then our application will be completely bypassed as the Add to Cart button is never actually clicked.
Here is a brief article by Shopify outlining this issue:
Unfortunately, this app is not compatible with buy now or dynamic checkout buttons. Nor is any app in the Shopify app market which offers similar functionality to ours. The buy now/dynamic checkout buttons are inherently flawed, in that they do not offer any solutions for third-party applications to work in conjunction with them.