I believe staples is using polymers. Most of the other printers would not be suitable or are much more expensive to purchase and operate. I guess the energy consumption of the metal printers is pretty extreme.
Not only not cheap, but another important difference is all those methods require some sort of minimal skill/research and some tools at least.
I never thought about making a ceramic lower before. I could probably manufacture a pretty strong one without too much problem. But it just so happens I studied ceramics for a year under the tutelage of one of the PHds who designed the ceramic heat shields on the space shuttle and I already have a kiln.
There are 3D printing blue prints available online to download. You could take them to a Staples on a thumb drive and hit print and out pops a lower receiver.
Some people have looked at printing full guns and my understanding is CSI was spot on about the barrels. Tensile strength is not high enough to stop deformation no matter how large the diameter. Of course, you could make a quick change barrel design or simply go smooth barrel route. You could probably print a form and try to cast a barrel.
Was thinking about things outside legal and realized you could likely also print it with any design you wanted whatsoever. Each mm/x thick layer could be a different color.