Amazon Faq: How to add an item to your existing open order.

To combine orders:

1. Add the new item to your Shopping Cart and select "Proceed to Checkout" to complete the order.
2. Visit the Order Summary in Your Account.
3. Click the "Want to combine orders?" button. This button will only appear if you have two or more open orders that can be combined.
4. Click the check box next to each order you wish to combine and select the "Combine orders" button when finished.
5. If the orders you're combining have different shipping or payment options, we'll ask you to confirm these options for the new order.
6. Review the new combined order and click the "Change" button next to any information you want to edit.
7. Click the "Confirm Order" button to place the new combined order.