You should make a option to load your payment options and then be able to select what option you want to use.
To be honest I don't need 100's or invoices I just want one a month I just have to pay.
I just wanted to provide an update on this Idea. This has been implemented and you are now be able to use existing My Post Business account as your payment method.
You can read more about this in the following Help Center Article:
Australia Post MyPost Business: Connect a MyPost Business Account to ShipStation
Thank you again for your idea!