The shopping cart for your order is stored in a cookie in the web browser you used to create it. If you need to order a single sample door, simply use a different browser on the same device, for example, Chrome, Edge, or Firefox.
Keep one browser open for your main order and use another browser for the sample-door order. You can also place the sample door order from your cell phone.
If you use Chrome, just make a new Incognito window and order away! The cart cookie is awesome and important to have. I’m in the process of ordering over 100 items. If it wasn’t for the cart cookie, I would be pulling my hair out since I can’t possibly order everything in one sitting!