Had to look this up. Looks like there was an earlier ruling in 1969 about sales tax on mail orders. The 1992 case seems later further refine the ruling.
Basically in Bellas Hess, the appellant claimed that the tax was unconstitutional on Commerce Clause and Due Process grounds. Specifically, that states imposing a tax on mail orders infringed on Congress's power to regulate interstate commerce and alternative that it violated appellant's rights under the 14th amendment to equal protection under the laws. In Bellas Hess, the Court agreed with appellant without justifying the decision explicitly on due process grounds. In Quill, the Supreme Court clarified that it was deciding on Commerce Clause grounds.
The relevant distinction is that Congress cannot pass legislation to allow the states to violate the 14th amendment. It can, however, pass legislation that allows the states to burden interstate commerce since if Congress has sanctioned it than any such state legislation doesn't infringe on Congress's power.
http://en.wikipedia.org/wiki/National_Bellas_Hess_v._Illinoi...
http://www.infoplease.com/cig/supreme-court/sales-taxes-mail...