The [ Buy / Activate This Software ] button will be present if you have not purchased and activated StarFilter Pro 4 and are running the plug-in in the free trial period. Clicking on this button will take you to the Software Purchase Activation Dialog where you can buy this plug-in.
After you have purchased StarFilter Pro 4, the [ Buy / Activate This Software ] button is not seen; the [ Help And Support ] button will be visible in its place.