Yes and no. You can lower the quantity of the item ordered, but you can not add additional items.
As long as your shipment is in the “Ready to Process” status or before, you can remove items from it. Here are the steps to do so:
1. Search for the order number on the Fulfillment Requests page.
2. Click on the fulfillment request id hyperlink
3. Click on the hyperlink next to "Order ID" under the "Fulfillment Request" header.
4. Click on the hyperlink next to the shipment that is "Validating"
5. Find the item you wish to remove or reduce.
6. Click on the blue number hyperlink under the "Requested" header in the shipment section.
7. Reduce the quantity to the desired amount. Note: Setting it to 0 will take it off of the shipment completely. All items removed this way will show up in their own canceled shipment on the page.