Using FSO to dynamicaly include a file only works if you aren't sharing any variables within the included file. A good workaround is using select case like this:
<%
SelectPage = Request.QueryString("Page")
Select Case SelectPage
%>
<%
Case "Login.asp":
%>
<!--#include file="../ShoppingCart/Login.asp"-->
<%
Case "MailPassword.asp":
%>
<!--#include file="../ShoppingCart/MailPassword.asp"-->
<%
Case Else Response.Write "Access Denied"
End Select
%>