Our Custom Product Options app works by hooking into the existing Add To Cart functionality to pass your product options as line item properties. And we interact with the cart by pushing any Add on items that the customer may have selected from your product page.
The Buy Now buttons bypass the cart entirely, and do not use the native Add To Cart functions. Which unfortunately means that any addon items that were selected cannot get passed into the cart to be sent into checkout. Nor will any required field validation fire, so required fields would not need to be filled out if the Buy Now button is 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.