On the premium add-on pages, for example Flexible Content, in License section stated:
You may include this add-on in a premium (not free) WP theme or Plugin
And in T&C:
Using ACF in a plugin
Including the (free) Advanced Custom Fields plugin inside a free / premium plugin is allowed.
You can NOT include any purchased add-ons within the plugin.
For your plugin to use any of the premium Add-ons you must ask the customer / user to purchase and include the Add-ons.