Checkout – adding optional product to checkout page

Hey everyone, i have a question that i cant seem to find the answer anywhere i look. i have tried searching through the forums and reading a few hundred posts, but haven’t been successful. Basically the question is, during the final phase of checkout, before i send the person to pay with paypal, i would like to show an additional product (a recommended product) to go with the purchase. The site is up and running, it processes everything correctly, now i would like to have my customers to see that we make a specific product to be bought in conjunction with the audio equipment they are buying. Has anyone ever done this, or gotten it to work? https://www.upgradecompany.com is the site, and would like to add that option to the final page of the checkout. or if possible to have it show up when they view their cart in general.
Thank you for any advice, ??