Already said offline payments are for ex-sin. Offline payments cannot use credit cards.
It is correct for that offline payments can only be used for itineraries ex-sin. you can use offline payment to pay for a round trip, sin-tpe, tpe-sin on the same itinerary but you cannot use offline to pay tpe-sin for example.
However, you can use credit cards to pay when using offline payment.
Use Vpost to pay for it.
vpost.com.sg
Vpost will prompt you for the booking reference and all.



