The attached sample plugin shows how you can use a Desktop Alert rather than a messagebox to alert the user. The Desktop Alert may be more suited to the messagebox when you don't want to interrupt the users workflow, but you want to provide some important information.
The attached sample shows an alert when the user edits the sell price of an inventory item to be less than the minimum sell price.
You can customise the appearance of the alert - including the icon, the size and position.
Mike