Welcome to WebmasterWorld rooze, If I understand you correctly, you want the URL to switch to the .com version when the user clicks checkout.
Technically this could be accomplished using htaccess but before doing this I would ask the question of how does woocommerce track the items in the cart? Will the user lose their cart contents when the switch is made?
Which also leads me to ask, why are you using .au? Why not just focus your marketing efforts on Australian resources and stick with the .com?