I just tell them "I need the FAMOUS ford starter relay on the fender." I've been driving fords for 14yrs and I think Ive replaced the relay on every one of the 10 fords I owned. If they sales guy doesn't know what your talking about after you tell them that, find another sales guy or another part store.
I also have another suggestion, although I didn't experience any clicking noises, I just had an issue with my 2000 Excursion. I thought it was the starter relay. Truck wouldn't start but I could jump the terminals underhood and it would start every time. I thought it was the FAMOUS relay so I replaced it. Turns out it was a steering column issue. If I tried to start in park, it was 50/50 on it starting. If I placed it in nuetral, it started every time. If you haven't fixed it already, next time it does it, slip it into nuetral and see if you just get the same click or if it starts.