Ken,
ST will refund your $20 if the other party doesn't participate in mediation. It happened to me - buyer never responded to me, I filed NPB and FVF credit and then they negged me saying I'd never contacted them. I filed with ST and the buyer responded to the initial case inquiry from ST and then it automatically went to a mediator. The buyer never responded further to the case in the mediation stage so I kept the neg but the $20 was refunded. If they respond to the mediator at all, my understanding is that it still costs you the $20 and you keep the neg.
Andy,
If you really don't want to live with the neg, then of course, pull the contact info first, but go ahead and file a ST case. Worst case scenario is you'll lose $20US - if you're selling on eBay long enough you'll deal with the odd neg and chargeback so you may want to set up a fund to cover those things, for example by adding 10 cents to each transaction (amount depends on your volume of sales). If they don't respond to ST - which they may not if they didn't even respond to your emails, you'll get the neg removed. Just make sure you do it within the time limit - I think its 90 days. After that, if they don't respond you'll have to keep the neg (but you won't be charged the $20). They would have to agree to it to get it off after that time limit has passed.