Once the order is sent to your POS we cannot change it. Your POS does not support our making changes to an order once we have sent it .
If you are capturing funds at checkout the order goes on to the POS very soon after that.
So under those circumstances, no, we cannot support order editing.
We MAY (and we say may because it is new) be able to support order edits if you capture your funds when you fulfill the items on an order. So, when the order is placed the funds would be authorized but not captured. So you could edit the order at this time. But at the time of fulfillment, the line item on the order is closed and is no longer editable. We would ONLY send the order to the POS after you have fulfilled and captured, after which there ca be no further editing on an order in Shopify. However these orders will NOT hit the POS until the day you fulfill them.