Peter. This patent was filed in 1998. It was reexamined in 2010. You're right that everyone is using it now, and it can be proved that Amazon used it before this patent was filed. However, someone will have to litigate this to bring that prior art into play.
We also received the letter. We have several attorneys working on this right now. One attorney we just learned of is Sid Leach in Phoenix, he specializes in patent law and has spent considerable time dealing with patent trolls. Our attorneys will be contacting him in addition to the work they are already doing. Many, many companies have received this letter. Our current take is that the patent will ultimately be proved invalid, but that is yet to be decided because it has not been litigated on its merit yet.