If your order hasn’t begun processing yet, you can add additional items to your order from the Order Summary Page. Instead of the ‘Order’ button, existing orders will show ‘Add more items’. Selecting this will let you edit your existing order.
Follow the below steps:
1. Go to your order (either from the home page or from the receipt section).
2. Click on “Add more items” on the Order Summary page.
3. Select/add items to your cart.
4. Once items are added, click on “My Items”. You will be directed to the My Cart page. Click on "Continue" to complete the process (you will need to reconfirm payment and other flows).