If you are in the US and trying to checkout with just extrusion in your cart, you will get the following error message: "Due to regulations, single extrusions cannot be shipped to the United States. Please go back to the cart step and adjust the items.".
This is because we cannot ship extrusions alone to the US (as parts), they have to be a part of a design. To work around this, you simply need to create a design from your dashboard, add the extrusions you need to the design space, and checkout from there.