Sending via UPS or FedEx internationally is almost an automatic hold up at customs. You can expect an import tax randomly and most customers who are charged that tax from customs will refuse the package. Australia, as well as countries in Asia and Europe, see import tax as a stream of revenue for the government and pursue it aggressively.
With USPS they assume the delivery is domestic mail, and not business related, and rarely ever check
Get hit by customs a few times and realize what an extreme pain in the ass it is, not to mention loss of income
