If a user specifically uses a feature that asks for notifications about a product, then you're just doing what the user asked you to do. The problem comes in when sending such notifications without them being requested by the user, which includes if the user "favorited" a product but that "favorite" operation had nothing to do with requesting promotions of that product. ("This is a thing I like" is not an invitation for spam about buying it; "Help me buy this" is an invitation for desired messages about buying it.)