Thanks to those who replied. If I do pay the ~$540 by GPMC, I will let you guys know if I faces any issue/s.
Managed to pay ~$540 by GPMC. The retailer insert the chip/card into the reader, and I got the amount deducted on the spot as reflected in the App.
Also get additional 5000 pts for the "GrabPay Power UP Challenge"
Not bad .....