Go to Cart Setup -> Advanced Settings -> Upselling Add-On

Add New Product
Add info for the new product - setting it to Main Cart page will add it to the main checkout page of Mal's

Edit item info

- Name the addon/upsell item
- Add the price of the item
- Leave as is unless your cart is set up with multiple currencies
- Add shipping units ( weight ) if needed
- Add tax rate if it should be taxed
- ( optional ) Add SKU if this items has one that you use
- ( optional ) If this item should only display - be available - when a certain item is in the cart add that item's Scode. Select "Begins with * then in the form field add the Scode of the item that should be in the cart for this upsell item appear. To find the Scode needed view this tutorial => https://merchantmoms.com/knowledgebase.php?action=displayarticle&id=173
- ( optional ) Add the URL of this item - navigate to the item in your store and copy the address from the browser address bar - paste that here
- ( optional ) Add the URL to the thumbnail image of this item if you want a thumbnail image of it to appear in the cart page ( see instructions below on how to find the URL )
- ( optional ) Set the size you want the thumbnail image to be if you added the image URL in step above ( #9)
- Set the text you want to show which will be what the customer clicks on to add this item
Click UPDATE PRODUCT
URL TO THUMBNAIL IMAGE - how to find it
PC - Navigate to the item on your site, right click on the image and choose "View Image" - copy the address in the browser's address bar
mac - Navigate to the item on your site, Ctrl-click on the image and choose "View image" - copy the address in the browser's address bar