At the JetStar website, I found that I could avoid the service fee if I paid by JetStar credit card or at the POLI service.
I tried POLI and found that it is an online service operated by the Australian Post Office but they require an account with an Australian bank.
I tried to apply for a JetStar credit card (co-branded with Master Card) but found that it is available only to Australian citizens.
Bad luck. I have to continue to pay the $18 service charge. The Australian citizens have two options to avoid this charge, but not the Singaporeans.
I wonder if the Australia government imposes a regulation on the airlines? Or was it just commercial pressure? Whatever is the answer, the Singaporeans are neglected.