I'd say DO NOT cancel the UPI case. Let it run its course.

After 4-days close it and get your fees back. And they get a strike which other sellers can use to block them if they have their settings right.

If they leave you NEG and still do not pay then after the UPI case closes without payment you should be able to get ebay to remove the NEG.

They could pay before the 4-days is up. But seems unlikely from what you are saying..

If this happens you will then have to ship their items and you stand a chance of getting a NEG that you may struggle to have removed.

Also make sure you add them to you BBL to make sure they cannot bother you again.

Once its all done you can relist the items.