You can buy any model by going to the model's page, selecting the material you want, and clicking the Buy Now button. This will add the model to your cart. Whenever you are ready, you can click the Cart button to checkout.
To access your cart, go to https://www.shapeways.com/cart or click on the Cart button on the top right corner on any Shapeways page and then select Edit cart:
This will bring you to an edit screen, where you can update the quantity and completely remove any models: