I would not answer your questions, just will propose you some workarounds.
1. Instead of using Session and Response.Redirect, why not use the Viewstate and Server.Transfer? Yes I know that viewstate is not encoded hardly and travels back and forth to the client browser, but you can encode the userid, before adding it to the viewstate. Note that when you use Server.Transfer you may have access to properties of previous webform from the current one, this way preserving the USERID. (You should add a property that will get and set the USERID from/to the viewstate for each of your webforms.)

2. What about (re)adding USERID to session object, just after the assign of VFPCOM_RCW instance to null e.g.

string usersig = Session["USERID"].ToString();
Pricer VFPPricer = new Pricer();
decimal YourPrice = VFPPricer.GetPrice(usersig, quantity, color);
VFPPricer = null;
Session.Add("USERID", usersig);
cmdSave.Enabled = true;

HTH