I think that you don't HAVE to go through the upi thread. Of course if you don't, you won't get your final value fees back. I also think that it is a good idea to go this route just for the strikes the would-be buyer gets. I do put in my auctions that I require payment within 10 days, but truth be told, I give more time. If I don't get my payment within a week, I file the upi just so they get the reminder invoice within the 10 day time frame and have a chance to pay. Then I take it from there on a case-by-case basis. I'm very easy to work with if you dont tick me off LOL... If you don't want to use the upi though I think you don't have to, maybe hmmm I've been wrong before but look here and see if you see what I see... http://pages.ebay.com/help/policies/user-agreement.html
I see that it says that "While using the site, you will not -fail to deliver items purchased from you, unless the buyer fails to meet the posted terms, or you cannot authenticate the buyer's identity;" So, to me that says that if I post in my auctions that items have to be paid for within X days and they are not, then that frees me of any obligation I have to sell them that item.