Curious, is the both ways shipping an Australian warranty requirement?
Australian Consumer Law (ACL) is pretty powerful and serious stuff. I had to know the previous Trade Practices Act (1974) and the Sale of Goods Act pretty much inside out, as we had a number of retail/wholesale establishments over the decades...
ACL is essentially a rolled-up, modernised and way fairer (to the consumer) way of laying out and providing clear responsibilities, remedies and requirements when it comes to the provision of goods and services in this country. Cross border (state) regulations where various state jurisdictions were making it difficult for agrieved consumers to get fair and reasonable service or even pursue remedies went out the window. Fairness now prevails.
Basically, the contract for purchase/sale is between the buyer (the person or entity who hands over or authorises the money transfer) and the seller (the person or entity that accepts/receipts the money). Warranties/remedies exist regardless of what a retailer/supplier/manufacturer states and no manufacturer conditions can extinguish certain warranties, requirements or implied warranties under ACL. For example: Removing a seal sticker or cover from a product, cannot extinguish a warranty.
One of them is costs for return. The retailer or supplier is responsible for any and all costs in relation to goods which fail the basic tenet of being not fit for purpose, not the same as the sample shown, have what is deemed a major failure, or a failure the buyer would not have purchased the product knowing the issue was likely in the first place.
I can for example return a Belkin router to a retailer where I bought it, and it is their problem to rectify. They cannot attempt to pass the buck, or palm me off to the 'manufacturer' or 'service agents'. My contract was with the retailer. Their contract was with the supplier. The supplier's contract was with the importer/wholesaler/manufactuer. None of that is my problem. Anytime within the statutory, or otherwise implied warranty (open to interpretation) we can simply march into the retailer and they have to solve/sort out/organize repair or refund/replace the product. Retailers will always try to pass the buck, but savvy consumers know their rights.
Return freight is part of that. A consumer cannot be held resposible for return costs on a product that was faulty or failed within a reasonable period or failed any of the basic fit for purpose requirements in the first place.
I know, a long post, but I'm trying to lay out an answer to your question as best as I can, without straying into the area of legal advice, which this is clearly not. This is just my experience, over many decades in this area and take it for what it is.
Where sales are made internationally, into other markets and freight costs are borne by buyers, the entire landscape would change and I don't know where one set of rights start and another stops. I would say March Audio would most likely bear all the costs where a product has failed through no fault of the consumer and is within the normal warranty period. Anything else would be unreasonable IMO.
