Well the fastest and dirtiest way is to add the script to your header or footer of the module settings.

Another way is to add an HTML/text module where you want it on the page and select basic text box/HTML then paste the script for it there.

The last easy way is to add it to your own vendor account for "inhouse" stuff like that, then add a banner module to the cart page and call it through the settings.

there are probably a hundred more options for you, but these are the most straight forward with the least complications.