I used ECIC, but not RST, so I can't speak to that. I was pretty confident in my technical knowledge, but I did use "Ace the Technical Pilot Interview" by Gary Bristow, and spent a little time reviewing some basics out of the PHAK, Aviation Weather, the Instrument Procedures Handbook, and a few other staples of aviation knowledge. Don't stress about finding and studying the exact reference materials they list out on the website. Many of them are military reference books and very difficult to find, they go into way too much detail, and others are simply outdated. That list hasn't changed in decades even though the test has. Use RST and other reference materials that are equivalent in their subject matter to the list FedEx gives you and you'll be fine on the job knowledge test.

In terms of interview gouge, get your hands on the Delta job knowledge tests and study guides. Those are the most beneficial. FedEx makes you sign a statement before you start the interview saying you won't divulge the material on the written tests, etc. So, the best advice I can give you is study as if you were interviewing at Delta and use those materials floating around the internet.

As far as ECIC goes, I went through the web-based videos and such one time, pausing and taking notes as I went. Then I did three webinar sessions, a practice SBI, and top-off session. The first webinar was very good, and the second was even better. I would do at least two webinars in addition to the SBI and top-off. You'll pick up on things you missed in the first one and get more out of being able to silently critique other participants your second time around. I did three, but didn't get much out of the third session. At least for me, after two webinars I hit a point of diminishing returns. The practice SBI and interview top-off are essential though.

If you don't have it already, FedEx will send you a ton of paperwork to fill out. I think most people print it and fill it out by hand. Personally, it was quicker and easier for me to fill them out electronically in Adobe Acrobat. In either case, you'll spend a few hours on it at least. Make sure it's complete and accurate, and follow the instructions FedEx provides (there are some fields you should NOT fill out). Unless you're military, you'll also need a verification statement showing that you're registered for Selective Service. A quick Google search should get you that from sss.gov (it's free). Do the paperwork early! As you get closer to your interview you won't want to be wasting time filling out paperwork when you could be studying. Also, if you have any updating or cleaning up to do in your logbooks, get that out of the way early as well for the same reasons.