I'm going to take a very tax centric view on this. You're at risk of double taxation here and with US corporate taxes being even higher than Australian corporate taxes and then the risk of needing to distribute dividends from a US based corp and suffer withholding taxes then pay taxes again in Australia you could end up in a very unpleasant position.

If at all possible you're probably much better off keeping your sales manager separate and attempting to book all sales as much as possible to Australia directly and only paying a cost plus basis to the US employee.

You should be able to achieve this if structured properly so that you can have a US based bank account (a bit of a pain to set up but possible) and avoid both transfer and foreign currency conversion fees.

Feel free to contact me if you'd like to discuss further.