Thanks everyone:smile:
I got someone to fix it for me and they said;
Quote=milkyway
I found the problem. It was the tax setting in Admin Configuration > My Store > Display Prices with Tax > which should be set to FALSE (only calculate tax at the end). The reason is, your tax is set to Australia only (not all zones), so other people from different countries will confuse when seeing the prices change (decrease) after login, also your Australian customer might confuse since they told to be charged for tax, but can't see the details. And payment gateway like PayPal will confuse because the order value prices is different from the displayed store price so it throw error.
I tested both using International Address and Australian address, both works correctly. Even for the Australian address, it shows correct tax charges at the checkout page.
============
Additional notes:
By the way I checked all your store files and found nothing suspicious, except some inactive mod like Austpost. I recommend to remove and replace with original files if you plan not to use this mod, since this mod modified admin page quite many.
Also for PayPal Express, I think some setting inside the client paypal express only allow credit card payment, at least it happened to me, I can't pay without using my credit card.
So I tested with PayPal IPN and as expected it works smooth. Please check my new order number #5
==============
So I think now no more problem in terms of the store setting/tax, it should works even with PayPal Express.