I think you have the POST-redirect-GET wrong. The point is not to "avoid calling an in-memory function" (whatever that is). The point is to do whatever action you need with the posted data and then redirect the user to a different page. So that if all this goes well, the user may refresh that page as many times as he/she wants without the danger of posting the same data again and accidentally submitting yet another order or yet another posting.

If you stick all the data to the session or somewhere, redirect the user and process the data only once you receive the GET request, you've just killed any reason to do the redirection.

IMHO, of course.