I think the alignment shifts for party members are automatic. I don't remember ever writing coding to scan through players in a party and shift them individually. All I do is use the default Bioware script for changing alignment. The built-in function does the rest. I don't think I can change it, and I'm not sure I should if I could. Like you mention, you are who you associate with. If you're traveling with an evil character, you kind of deserve to become evil yourself. In life, if you associate with and work with a murderer, for example, you are also guilty of a crime.
As for making it so that players can pay to shift their alignments however they wish, that would defeat one of the core principles of DX/KT that players are free to steal and kill, but they are held responsible for it. I made the bridges, waitresses, alignment books, and other NPCs so you could periodically get a chance to alter your alignment. The changes are enough to correct an occasional mistake, but they won't fix a player who is trying to game the system by acting contrary to a particular alignment on a regular basis.
If you can think of some other methods to shift players small amounts on a limited basis, I'd like to add them. It's difficult for me to come up with law/chaos events. I thought maybe I could add a tax collector who targets players randomly. Pay and you shift lawful. Don't pay and you shift chaotic. Like I wrote, let me know if you have ideas to handle this that fit the game environment and aren't simply ways to set alignment any way one wishes.