Hi Sunshine
You can buy flight tickets on line on the day of the flight if you want to as long as you go direct to the airline so that e-tickets can be e-mailed to you. For that matter you can buy seats just by walking going to the various airline Information Desks at the airports.
However, do
NOT be tempted to try to reach Oz after the Initial Entry date in the Grant letter. They can be waived on compelling and compassionate grounds but nervousness is not one of those!
This won't seem nearly so bad once the plane is in the air and you can relax with a pre-meal aperitif and let somebody else wait on you hand & foot for a few hours, chooks. The build up is nearly always worse than the actual execution of Plan A.
Chin up. You're a survivor so you
will be OK.
Hugz
Gill