Since every transaction on The Exchange is between 1 buyer & 1 seller, it's not possible to combine listings from different sellers into a single cart/transaction. However, it is possible to combine multiple listings with the same seller & storage location into a single cart/order. Here's how:

During checkout, we'll display additional listings from the same seller. Simply click on a listing (or View More!) prior to entering payment information to continue browsing:

From there, click Add to Checkout if you want to buy, or just click on the shopping cart icon at the top of the page to return to your cart without adding the listing to your order:

If you've already made multiple purchases from the same seller, there's only one way to combine those: Cancel all of those orders, then re-order by following the steps above. It's extra work but will save you a lot on shipping cost.