I need to charge for a consulting services, just once. What is the cheapest legal way to do it?
Is ABN the only option?
UPDATE:
Got ABN for Free (yay!)
https://www.business.gov.au/info/plan-and-start/start-your-b…
used this template
http://www.intuit.com.au/r/money/free-download-invoice-templ…
Can you please recommend me cheap tax agent (Sydney or online)?
Yes. Getting ABN number is free and easy. Follow the guidance here: https://www.business.gov.au/info/plan-and-start/start-your-business/business-and-company-registration/register-for-an-australian-business-number-abn