Hm, I figured out how to get that code into the cart, but it didn't actually execute that script to display the banner, probably because it's only getting inserted into the page when the cart is viewed, rather than when the page is first loaded.
There might be some way of rejiggering things to get it to work, but I'm just not skilled enough in javascript to figure it out myself, and that's really beyond the degree of support we can reasonably offer.
If they offer a non-javascript version of the banner code, that should be much more straightforward to insert in the cart, since you wouldn't be trying to put scripts within scripts within scripts.