There are numerous third-party sites like this that offer Australian ETA applications as a service, which are often even cheaper than the official site. However, as of February 2023 the official site now says:
Important information about the ETA
All ETA-eligible passport holders must apply for an ETA using the Australian ETA app.
However, the third-party sites still seem to be operating. Is it still possible to use them, or do you really have to use the app?
For context, the third-party sites used to work by integrating directly into airline GDS systems, while the app now requires taking a selfie and scanning your passport's NFC chip.