I have never sold on Ebay, but here is my 2 cents worth.
I think you have to wait to offer the 2nd chance to the next bidder until 7 days have passed, since ebay allows buyers 7 days to pay. Many people pay immediately, but with the 4th of July holiday, maybe she was out of town and unable to respond to your email. I would wait to email her again. If she is not intending to pay, a second email isn't going to make her, and since the auction just ended a few days ago, she might see it as harrassing to have payment demanded twice while she was still in her 7 day window. If you want, email her on the sixth day reminding her in a nice way that she has just one day left to pay before you would be forced to report her as a NPB. But be nice! You will catch more flies with honey than vinegar, and it doesn't cost you a thing to be nice. Either she will or won't pay and getting mad or threatening her won't change that.
However, it does appear that the bidder may be waiting to see if she gets the same item cheaper elsewhere. If this is the case, wait the 7 days, report her as a non-paying bidder, block her from future auctions and offer the 2nd chance to the next highest bidder.
There is also the matter of feedback. You should go to the discussion boards at Ebay (click Community, then discussions boards, then feedback) and ask the boardies there whether you can leave FB for a NPB. Again, if you leave FB, don't get mad. It just makes you look silly. Be factual--something like--NPB, never resonded to emails.
anyway, I hope this helps and good luck resolving this.
