For bills? I do all the time (I also use the online bill payment service - some you can pay with PayPal, some you can't). Australia Post don't charge anything to pay bills there, but the biller often does. Check the bill, it should outline fees and charges - if any - for various payment methods, and they're usually added to the next bill (Optus charge fees to pay your bill for every method except direct debit 😐 ).