Unable to set planAmount for Charge 2 - Stripe
Developer of Charge here.

This error is because you're passing an invalid currency code. 

You're passing aus as the planCurrency, but need to be passing aud for Australian Dollars. 

Under the hood, Charge is seeing that currency code as invalid, and unsetting the amount from the request, which is why that error is showing up on the planAmount. 

(Charge unsets the amount as a safety mechanism to prevent the request falling back and processing in the default currency)

I'll update the language of the error to make it clearer the currency code is the invalid value in an upcoming release.