Unfortunately, the answer is no. For security reasons, we don’t allow restaurant employees to make edits to confirmed orders aside from issuing refunds.
If the customer needs to add a tip, then the easiest fix is for them to tip in cash when the order is delivered.
However, if you really need to add a tip to an order (i.e. the customer insists on tipping via their credit card), then you can refund the order and ask the customer to submit it again (with the proper tip this time). This is very easy for your customer to do from the “Order History” section of their account page – it will literally take them just a couple of minutes.
Hope this helps!