Firstly, order your printing (letterpress, digital, foil, whatever you fancy really) in the size that you require. For instance - if you want a 120x180mm card printed white ink one side, order that and choose your stock.
Add that to your cart.
When the page refreshes, scroll down and you will see an option for Edge Foiling or Die Cutting under the image.
Click on Die Cutting and you will be taken to shapes relevant to the size printing that you ordered. Choose the one you want and add that to the cart too.
They don't however need to be all cut from the same stock - say you have ordered 60 x diecutting, you could have this made up of 3 different designs on 3 different stocks with 20 copies of each if you wanted to - just so long as the size is the same and the quantities add up. (This is a great way to sneak in some die cut samples if you wanted to!)
For shapes that are too big for our standard envelopes, they're ordered a little differently:
Printing by Type > Printed (larger) shapes > Select your stock in the drop-down.