Need a Business Account with Australia Post and you will get a discount.

You can use  "POSTAGE PAID AUSTRALIA" or "POSTAGE PAID AUSTRALIA - PRIORTY"

on the letters

I use a preinked stamper (though I am now adding the postage paid mark to my address labels - using a brother 570 label printer)

pre-ink stamp I got here: http://stores.ebay.com.au/EBPACK/_i.html?_nkw=stamp&submit=Search&_sid=1012747416

"postage paid Australia" aka as surface mail can take an extra day or 2 compared to normal mail (stamped or "priority mail")

Large letter up 125gms I pay $1.24

Large letter up 250gms I pay $1.86

Large letter up 500gms I pay $3.10

Priority mail: (equivelant to using stamps)

Large letter up 125gms I pay $1.36

Large letter up 250gms I pay $2.04

Large letter up 500gms I pay $3.40

(Before July 2014, there was no discount).

NO MINIMUM. Send 1 letter if you want...

more info:

http://auspost.com.au/business-solutions/metered-imprint-mail.html

Cheers