By ebay's rules the buyer has a full 7 days to pay. That is why you have to wait that long to file the unpaid item dispute. I would give it that long at least...then file the UID and relist.
In fact...I think it's a no-no to relist within the first 7 days because they have that long. You can sya in your auction you want payment in 3 days, but that is against ebay's rules.
I just had one who had 100% feedback, I had to file the UID and she paid 2 days later saying she went out of town and totally forgot!
I'm sure it's the same thing happening to you right now!
Also...send the invoice...wait at least another 2 days and then resend invoice with a polite reminder. After 3 days then I would send 1 invoice per day, and on the final day (7th day) say you're filing the UID.