Related products are one of the most important features of the Fast Cart. They can make a big difference to your average order value, bringing in more revenue from each customer.
The "You may also be interested in" section of the popup cart displays cross-sells for the product(s) currently in the cart. You can set these in the 'Upsells/Cross-Sells' section of the 'Add/Edit Product' screen for each product.
By displaying related products depending on the cart contents, you can encourage customers to spend more. They can then add one or more related products to the cart directly from the popup cart, before completing their order quickly and easily.