Dont fret too much on the 10% it was only on Additional Store items or Extra playfields,and only available on the combo order..
Bringin a second one in would put you over the 1k duty limit and then you would be paying 10% Gst!
I've ordered multiple playfields in one shipment. I've also ordered 3 backglasses and 3 plastics sets in one shipment. If you want more than one playfield, email Kevin and ask him if he can help you out with keeping the value under the AUS$1000 limit.