You can add up to 30+ items to the shopping cart and shipping is only charged once.

We recommend that after you upload the first set of files, you add your contact information as if you were completing your order, and once you’re in the shopping cart section, you’ll see a button near the bottom of the page that says “Continue Shopping”.

Because the site doesn’t know whether you are ordering the same product or ordering a different product, you’ll have to go to the product list to add your next item (either the Fast Checkout button or the Online Design button), and so on …

Also refer to How to Order for more information on placing orders.