Just wanted to chime in and say that I've started using these and wanted to provide my thoughts. Before I start, I should mention that Ebay has the postage for these at 50% off until the end of May, meaning that you can ship for $4.28 for this month only.
I ordered 300 of the mailers in preparation. The bulk rate ended up coming out to 25c a piece, only slightly more than I'd pay for a generic padded mailer of the same size. The first thing I was disappointing about however was said size. They initially only provided a single measurement: 280mm x 215mm. This matched a bunch of generic mailers I was using so I was initially excited. It was only after they arrived to me that they updated the page on the AusPost shop to include the internal dimensions which are 270 x 180mm. This is quite a bit smaller and was very disappointing.
In my experimentation over the last few days however, I discovered that the edges of the mailer are actually glued together and not solid paper. This allows you to easily peel them and by doing so, you can actually fit far more inside than you might think. I've sent about 20 packages so far and I'd say about 12 of them I've only been able to fit by tearing and taping the mailer back together to essentially increase the internal dimensions. Before you ask: yes this is fine to do so long as the entire article is enclosed when you send it. This has been a huge game changer and means they are actually useful on a large scale to me now.
As for the mailers themselves, they are made purely out of paper with some kind of corrugated cardboard cushioning instead of plastic bubble wrap for padding. This feels surprisingly sturdy but unfortunately isn't waterproof, which as a bookseller is quite the concern. I do think this can be solved by wrapping your articles in shrink wrap to an extent, but it is a downside. The internal paper also tears very easily when trying to insert articles that are nearing the size limit for the mailer, but this can be entirely mitigated by using the tearing method I detailed before.
As for the postage cost, there was some speculation here that the $8.55 rate advertised would be the Zone 1 (40% discount for Band 5 on MyPost) rate. Fortunately this doesn't seem to be the case: $8.55 is the base 0% rate. If this ever stops being an Ebay exclusive and comes to MyPost (which I very much imagine it will down the line), this will finally be a welcome relief for sellers of smaller items as we'll be getting a 5%, 20%, or 40% discount (Band 5) on that $8.55.
The mailer follows the same standard 5kg maximum weight flat rate rule, although my mailers (and apparently many others) have a maximum weight of 250g printed on them. I assume that was Australia Post's original plan before they wisely decided to make them 5kg to match the other flat rate sizes - I have no idea how they even entertained the idea of 250g as a maximum weight. If you get yours and they have the 250g weight listed, they can still be used normal with 5kg maximum weight, but you might want to let the Post Office people know when you drop them off as most are not even aware that these new mailers are a thing.
Overall I went from disappointed to quite pleased with these mailers after figuring out the tearing/taping method. I'd highly recommend you try and pick some up at the very least to take advantage of the 50% off shipping until the end of May. They'll only offer a paltry 21c difference to Zone 2 shipping (Band 5) vs The standard Small Flat Rate after the promo ends, but hopefully Ebay will still provide the 10% off postage they initially advertised in their Emails so they continue to be worth using until they come to MyPost.
