No, you may not amend an e-SHOP order. This will cause an error.
If the order is still in the requisition stage, you may cancel it yourself and create a new requisition to include the change.
If you wish to cancel the entire Purchase Order (PO), first, contact the vendor directly to let them know, then request that your BSC void the PO.
If you have already received part of your order and wish to cancel the remaining items on the PO, you must first contact the vendor directly to let them know, and then request that your BSC close the PO.