i shopped around a bit, it appears there is not much variance in pricing on givi, i wonder if there prices are fixed by givi?
highly recommend sporttour.com. i'm lucky to live about 10 miles from them. they are great to work with. they also have great prices on tires.
it would be my strong recommendation that you opt for the givi monokey rack to mount your givi topbox instead of an aftermarket one. i had one, and although it was better looking, larger and more usable for putting other stuff on it when the givi wasn't on it, it also was a flawed design compared to the givi. it cracked my subframe, as it puts too much stress on the wrong areas. mount the givi rack and that thing is _rock solid_.
i had a recent "adventure" on my fjr resulting in rashing up my E52, and got some insurance money to replace it. i shopped for the 55 liter like you are, and saw some questionable reviews on its design and function, so i'm sticking with the E52. just a heads up. good luck!