The 2 Peso per liter discount during the holiday season is limited to Metro Manila only although some stations outside MM offer it also.

Shell NLEX does not give the 2 peso per liter discount, not sure if Petron NLEX does.
Better gas up in MM before going North.
BTW, Shell in Dau Mabalacat have the 2 peso per liter discount.
As a result, their prices are now cheaper than that of Shell NLEX.