6ft extension cords you buy for 1-2$ are fine for flexwatt as they are 16g and handle 13amps/1800watts. No one will ever over load that wire or cause it to overheat.
True, but the heater cable the insulation will not burn unlike the plastic insulation on cheap cords if the connection arcs the insulation will not catch on fire. That is why I suggest it. The small additional cost adds a small amount more safety.