How can I purchase more than one copy of this item? I am not able to edit the quantity field in the shopping cart.

If you find that the "quantity" field is not editable, then the company that offers this product has restricted the number of copies that can be sold in a given order to be 1.  You can contact the company directly to see if they can address this issue for you.  Their contact information can typically be found on their website or in a "contact us" area on their site.