RTS can be a pretty risky way to do a return, if for no reason other than Aus Post's sorting system is largely automated, so they will scan the QR and barcodes on parcels to determine where it needs to go (I had a buyer in NT send something back marking a parcel RTS - it was unopened as I had sent them the wrong item, I sent the right item the day after via express, and asked them to RTS the first one when it got there afterwards, which they did).
Anyway, that RTS package took 2 months to get to me (and this is pre-covid), because it just bounced around all over the place and was even re-delivered to the buyer. (It was tracked the whole time, though, from memory - they don't exclude RTS packages from the scanners when they go through sorting, that would require hand sorting which defeats the purpose of automated sorting, so if you look at the original tracking number - if there is one - it may show some scan events already).
If it was an untracked letter, then a bit of a different story, but in either case, as long as the markings are clear, and the original delivery address is crossed out, it should eventually get back to you. However, RTS parcels can sometimes attract return postage fees (charged before you can collect), so just be aware that might happen and maybe consider some diplomatic ways to handle it if it does, before you're on the spot and trying to think of a message / reply. (This doesn't apply to letters normally, but as mentioned above, the articles are meant to be unopened).
As far as the request goes - you'll start to get automated emails from eBay soon, "the buyers package should have arrived back by now, make sure you refund them ASAP". and other greatest hits. However, if you do not progress the case, it will eventually time out (unless the buyer escalates, but without valid tracking / proof of delivery, eBay is unlikely to decide in their favour).
