Ancestry no doubt have various "collection" centres worldwide, which receive completed test kits for their local region and forward in bulk to the processing lab at Ancestry's expense included in the cost of purchasing the kit). Ancestry do say in their help and support pages that return postage is only valid for the region in which the test was originally sold. Presumably if you live in Australia, have an Ancestry.com.au account and purchase a kit from Ancestry, it should come with prepaid postage to the local centre.
The problem I suppose might arise that a kit purchased from a third party such as Amazon, might be sold into a market it wasn't originally intended for. In that case Ancestry are quite clear that the test must be returned at the user's expense, as they also state is the case if the user wished to use a different form of packaging to that supplied, or use a different postal method such as tracked or signed for. The same would apply to a kit purchased for instance in the UK, and sent to a relative in Australia for example.
With regard to sending a kit from Australia to Dublin, with no air routes available, I suspect the postal service would choose to send an item that they collected from a public mailbox by the least expensive method. I do have some experience of similar with one of the parcels that I previously mentioned sending regularly to our daughter in South Korea. We always post them tracked. Last Christmas, a parcel that I posted to her was sent by Royal Mail by airmail to South Africa instead of South Korea, because the clerk in the post office had keyed in the wrong country when the parcel was accepted. From RM and international tracking, we could see that on arrival at Jo'burg distribution centre, it was readdressed in their system and sent by surface mail (shipping) to Korea, via Spain!!! My daughter received it nearly 4 months after we had originally posted it.
