Consider I have bought a WordPress plugin with extended support.
Does it mean the author should fix any compatibility issues with other free, premium and custom plugins/themes?
As an author what we should do when we don’t have access to another premium plugin?
Help with “included” 3rd party assets. If your theme says it works with “Contact Form 7” then it should work. If it doesn’t then you need to fix it.
If your theme says it works with WooCommerce and someone complains it doesn’t work with Jigoshop, then you don’t have to fix it (but it would be nice if you did).