Depends.
I deal with imports from Oz all the time, and have for the past 18 years or so.
You never know.
I've been "dinged" twice, appealed, won both.
What happens is you get a notice in your mailbox or from your carrier, stating you owe the Customs folks so much.
In my case (USPS), the postman didn't even have the package. I had to drive to the main Post Orofice and pay up to get the package.
There is no way I know of on INDIVIDUAL shipments to get a pre-determination.
Try to get some yay-who at Customs to even answer the phone.
Now, if you're bringing in a shipload of Toyotas every month, yeah, they'll tell you how much.
But they'll still inspect them.
They determine if the product is competing with US made goods.
They see if it can be used for another purpose than that stated (like your ecologically-friendly back-up battery for emergencies....if it can be used in an automobile, it is taxed on the higher use).
So, depends.
Also, on how smart the shipper is and what customs code he puts on the box.
Have fun!