You can add a link directly in the cart itself, such as a link to another product's sales page, by adding these lines to your View Cart code, just before the "// -->" line in the standard code you obtain from Seller Admin:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#tdPmnt").attr("innerHTML",
"Any custom <b>HTML</b> you want to show in the cart");
}
Since this needs to go on each page, and affects the cart only when it is displayed from that page, you can make the custom HTML you insert in the cart different for every product's page.
A fancier solution has been devised by one of our clients, MuvEnum. If you Add to Cart an item on their sales page, you will see that adds a link right in the cart to add a CD Backup upgrade item, and removing the original item from the cart also removes the CD Backup item as well. To figure out how they did that, you would need to examine their source code or contact them to ask for advice:
http://www.muvenum.com/buy-now/
