Just want to contribute some latest info here.
At the very first few pages of this thread, somebody was saying that we can apply for visa or ETA with Singapore Airlines (if we fly with SQ) for free or cheaper than standard rate. This may be true back then, but not anymore now.
I asked SIA office in Changi Airport, their ETA submission fee is S$50

, and they don't accept visa application (non-ETA) - don't bother about cheaper fee

So I ended up doing my ETA with JA Travel, S$10 each. Somehow most of other travel agents that I asked are currently charging S$20.
Hope this info is useful for those who are not eligible to use Ms Kang's service here