Unfortunately eBay doesn't seem to prioritise or care about this issue - I made so many requests for this to be implemented, reported every instance of mail that was RTS and involved in an INR (clear evidence of a disappointing buying experience for buyers), but they barely acknowledged it, aside from a "we'll pass this on to our tech team for consideration" 
Successful delivery of packages that aren't eligible seems to reply on so many varying factors, right down to the location and the whims of the staff handling the mail, but my fail rate for delivery was so high I just made it a flat rule not to send to them any more - I have a big red warning in my listings, but still get people supplying locker addresses - some are fine when I ask for an alternative address, some get mad, some never reply at all and I cancel the transaction.
You can't actually create a postage label through Aus Post for a parcel locker or collect address without including the buyer's email on the label, so clearly they have determined this to be an essential component of those addresses, and those can also often be on international packages regardless of the size - these labels are all created electronically, with data shared to Aus Post's systems automatically, so notifications and the like are more likely to reach the recipient - I wondered if putting the email on a letter package would make a difference, but no one at Aus Post is going to even see it, let alone enter it into their system, at least not until the final point where someone decides what to do with the package, and at that stage the email address isn't going to help any more than the other customer-identifying info on there (and which should already be in their system, anyway).
