Done! We have deployed the feature just now. You can choose to place buy or sell orders in both AUD and BTC. In most cases you should get the exact amount, but fluctuations can also happen in which case you can always cancel and create the order again. (This is a tricky feature to implement because Filler and Checkout platforms are entirely independent and they don't trust each other.) Thanks!
Personally I don't find the new implementation very helpful.
It'd be nice to be able to figure out the conversion rate without clicking the 'request quote' button. On other websites I am able to input BTC or AUD and immediately find out what I am going to be purchasing and for how much before clicking any submit buttons or making any actual deposits.