I understand your explanations. The client's site is a trade site so that when a customer is adding many, many individual items to the cart in one session the alert just provides definite confirmation of the correct use of the mouse i.e. "did I click the add to cart button properly".
Ultimately saves time by not having to periodically check the contents of the cart when ordering a large number of items.
Adding the onclick event was quick and easy to implement and works well. Many thanks.