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:

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.