I want to display a pop-up message to customers when they checkout in the webstore with particular products in their cart. Ideally it would be displayed when they hit submit at the checkout in the Web store... So, they would click on Submit (or checkout - whatever the final button is), and the message would pop up and they would have to click on ok on the message for the order to actually be submitted.

Sounds like it can be done. Has anyone done it and what's the best way?

The pop-up message should add on a whole category of products. And if we are going to build it to work for one set of products, it would be good to have the capacity across other products also