I've just found out after much angst that Powerstore and eCart can be set to charge flat rates depending on the country. e.g.
sub total of quantity = 1
if session variable exists [eCartCheckoutForm_country]
if session varable [eCartCheckoutForm_country] = AU
Flat Rate = $7
Simple really - when you know how!
I can now see that there are heaps of ways of setting shipping in Powerstore using eCart, it's just that the support documention and an overview of how the shipping works is lacking.
There are heaps of forum posts regarding shipping options so it appears that many people, including me, are finding this part of the system a bit confusing.
Please take this as constructive critisism! I think Powerstore and eCart are excellent products .... really.