What is the fare/tax breakdown of the $99 flight?
You can buy a gift voucher for $50, pay the remainder on your cc and bypass the $7.70 credit card fee. This is of course only cost effective if the fare component is $50 or more - ie. equal to or greater than the voucher value. This is because vouchers cannot pay for taxes.
Depending on your cc you can also get up to 2.5 pts per dollar spent on Qantas flights, services etc.