I've worked with Glass Cages on sheets of glass and have relatives who have gotten tanks from them. A few things to keep in mind:
1) They have a history of inconsistent quality. When I got a sheet of glass from them (100" x 24" x 1/2") it was spot on. I also looked at the other tanks in the van and while clearly high end they all looked very good. Certainly on par with most builders, and nicer than your typical Aqueon tank. On the other hand, the tanks my relative got from them (as in more than a dozen) had absolutely awful silicone work. Thick smeared seams with extra globs in places, silicone finger prints all over the inside, etc. Now, these experiences range from 2-10+ years ago, so they may have improved, but do some research.
2) They have already cut a lot of the costs out of shipping. When I've seen orders come in they had a well filled truck of their own on a reasonable route. They are depending on you to unload the truck, and meeting at a common location. As a result they are already sharing the cost of shipping your item with lots of other people (as is any efficient shipper) and eliminating a lot of the things most shippers charge for (residential locations, lift gate, etc). There is some economy to having more items dropped at one point, so it doesn't hurt to ask, but don't expect a big difference.
I estimate 9/10 times they are nice to work with, particularly when you keep the above items in mind. Just consider that they've already done a lot of the cost cutting for you (both in quality assurance and in logistics), which is why the prices are what they are.