Just canceled my PBB card.

I called in and ask them to waive my GST and they say it is impossible for them to do so. So I told them, ok, just cancel my card. Poof, she canceled my card that instance without asking any question.

I will call back later and check if the card is still active and try to use at a petrol station to see if the transaction could go through.

Call customer service to do so. (some requires you to walk into their branch but mostly don't) Then, try at petrol station to see if the card still works or not. This is at least what I did.