The Multi-item Card Add-On allows customers to purchase multiple products with one transaction.
By default, iThemes Exchange is set to for single item purchases. You can enable the Multi-Item Cart add-on in Exchange > Add-ons.
Once enabled, your customer will now see an Add to Cart button when shopping (button and cart styling will vary based on theme being used):
Once they've added the item to their cart, they will have the option to view their cart.
In their cart, they can make adjustments to their cart such as changing quantity of a product, adding coupons, etc.