you would do this in a calculation in the cart object, so that the value can be reset on an item by item basis.
you could create a calculation called "resetShipCalcMethod". for examples sake i will reset the ShipCalcMethod if the session variable "reset" equal 1
resetShipCalcMethod abs(isset($_SESSION['reset']) && $_SESSION['reset'] == '1')?[ShipCalcMethod] = '':''