They've paid out a lot of times with Authentication Guarantee as I'm in a few groups for watches and trainers and I've seen people worry about the tracking not updating, as long as its in the system eBay pay out. 

Last time I had to do a claim with Royal Mail they wanted a receipt not just an eBay screengrab, screengrabs can be easily doctored or easier than an official receipt I guess, but its been a few years (touch wood). eBay gives you the standard protection for special delivery too, its upto £750 which I always thought was standard special delivery. Anything above that though I'd be paying extra myself for sure.