A reader wrote in recently in response to my post on the comparison of the budget airlines in Singapore, highlighting an additional (hidden) charge of $1 when she tried to make payment at an AXS machine for a trip on Tiger Airways in February 2014. Booking using a credit card will incur a surcharge of $9 per person per way, so the cheaper option would be to pay through AXS, which Tiger charges an additional $5 per booking. So the booking amount that was supposed to be paid after completing the transaction online was $243.16. However the amount charged at the AXS machine was $244.16.
Apparently this is written on their FAQ here.