They can leave feedback even if you refund. If you do refund through ebay and cancel the transaction using the 'buyer requested' reason, you shouldn't get a defect. However, I doubt that I would refund now. I would explain that the item has already been mailed and that they should receive it in approximately xx number of days but that it is a busy time of the year for the postal service so it may take slightly longer than that.
If they do file an inr down the road, any defect that you receive will drop in February with the new system so unless that one defect is going to cause you significant problems, I would just wait and see how things go.