Is it possible to add a note to an order AFTER it is completed.
I'd like the user to add a note AND add data to a hidden field that is added to an order.
This really needs to be done after the order is paid for.
Store uses channel entries to store it's data, so theoretically you could return them to a URL with the entries ID in it like http://sitename/thanks/999
and make an index template inside a folder group title 'thanks.group>index.html' and make it a channel form that picks up {segment_3} as the entryID.
Hide the rest of the fields and show just the comments field, hitting save will save the entry with the extra detail. You'd likely want to do some serious checking/validation to make sure it's the same user/session as you don't want just anyone typing in random numbers stuffing other people's orders, but defiantly worth a look.
I've done similar with Freeform and I think post-purchase updates.