Can't speak to how this approach would 'cost compare,' however, I have dealt to my satisfaction with both your stated matters (fenders, et.al., + empty fuel containers) by simply giving my entire rig (bike, panniers, & then some) to REI at one location and had them pack/ship from one REI location to another REI location close to my tour start point. I have taken this approach at least 3 times and will continue to do so IF, of course, REI locations are in proximity to my tour start and stop sites. I saved more time on 1 or 2 of those occasions by paying REI to reassemble/test out my bicycle, as well. Coming home I always had the option of having them pack/ship straight to my house.
Last time I tried that they said the policy had changed. I forget the details but the price and logistics turned me off. If I remember correctly they were charging to box it, using UPS to ship it at full cost, and charging me to unpack it and assemble it on the other end. That last was mandatory.
That was in 2012 at the Denver store. I said "no thanks", got a box from them (which they gave me for free), boxed it up myself, and used UPS to send it home. Since I had a rental car at that point I dropped it off at a UPS store myself.
Have you shipped that way since then?