If you would like to wait until another time to buy some of the books in your cart, you have the ability to save for later.
Should you wish to save a title for another purchase, select the 'save for later' button for that specific title. It will then be moved to the 'Save For Later' section at the bottom of your cart.
To add a title from this section back into your cart, click the 'Add To Cart' button for that specific title and it will be moved up. However, if the book is currently unavailable, you can instead add the title to your wishlist to be notified when it comes back in stock. To do this, click the heart icon.