How to display warning/alert message on navigating to Add Product page by clicking “Add Product” button in the related list from opportunity detail page
You have to override the Add Product button that needs a custom visualforce page to diplay the message. You can follow this guide to implement it since is a bit difficult unlike other buttons override:
You can follow this guide to implement it since is a bit difficult unlike other buttons override:
http://www.michaelforce.org/2011/04/03/snazzy-product-entry-screen/