Shipping an exchange item, or a replacement for a faulty item?
These goods are still subject to Duties and Taxes, so you will need to adjust the product value in your order in ReadyToShip to represent the true value, so it is correctly declared for customs.
Duties must still be prepaid, as per the original shipment. Correctly declaring the shipment will ensure that these duties and taxes are correctly calculated by the relevant service used by your carrier (eg. Zonos via AusPost, Sendle DDP or DHL DDP)
$0 values are not accepted when creating a label, and falsely declaring as a GIFT for $1 is a Federal Offence. (see our article on this here)
If you are not prepared to cover the cost of duties for the item being shipped, you should collect them from your customer as part of your Returns and Exchanges process.
The above also applies even if the person receiving them is getting them for free in return for content or promotion (eg. Influencers).