It is not always possible to add to an order placed on the Amazon.com website. It depends on, among other factors, how quickly you act after placing the original order. Amazon orders are generally processed swiftly. If too much time passes between when your original order was submitted and your additional order is placed, the likelihood of your being able to add the second order to the first order is probably not great. If you just submitted the order, however, and would like to go back and add something else, this is not difficult to do.
Adding to an order placed with Amazon
Step 1
Visit the Amazon.com website.
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Step 2
Browse to find the item you would like to add to an unshipped, previous Amazon order. On the item's Web page, click the "Add to Cart" button.
Step 3
Repeat Step 2 to add any additional items to your shopping cart that you want to purchase.
Step 4
Go to the "Amazon.com/your-account" Web page and log into your account.
Step 5
Click the "View Open Orders" link in the "Order History" section.
Step 6
Click the button labeled "Want to combine orders?" (This button is only available if there are two or more open orders in your order history that are eligible to be combined. If your previous order has already entered the shipping process, for instance, you will not have the option to combine this new order with it.)
Step 7
Review the combined order summary. If any information needs to be modified, click the "Change" button and edit the information.
Step 8
Click "Confirm Order" to submit the new combined order.
Modify an Amazon Merchant Order
Step 1
Go to the "Amazon.com/your-account" Web page and log into your account.
Step 2
Click the "View Open Orders" link in the "Order History" section.
Step 3
Locate the order you wish to modify. Click "View Order."
Step 4
Select "Problem with this order?" Click "Contact Seller."
Step 5
Send the seller a request to modify your merchant order.