You can set an overlay to be displayed when a specific event happens in a shopping cart of your visitor (e.g. number of products placed in the cart, or some specific product is sent to the cart). This can be done via cookie targeting. In order to use the option, please, check the article about using a specific cookie for a campaign.
Make sure that when a specific event happens in the shopping cart of your visitor, the visitor receives a cookie. Setup your campaign to display the overlay only when this cookie is present. This can be done by setting up Cookie name and Cookie value.