And your Australian customer will be charged Aussie sales taxes and possibly duty on his purchase.
You don't have to know what that cost will be, but when you send him the shipping cost, politely mention that he will be paying those costs on arrival before he can claim his purchase.
If he balks, offer politely to cancel the transaction.
He may not be aware of how much imported goods cost over the basic price.
https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1
Politely-- Captain Malcolm Reynolds.