Call me thick, but since Monday eBay no longer offer a postage service. Would that have something to do with it?

It might well be that once their wunderchimps have worked out the new postage costs (unlikely given that all 500g eBay listings have been defaulted to medium, not small, parcels) they might well be able to reinstate eBay postage labels.

Or not, given that might require them to employ people who actually know what they're doing. eBay Australia is renowned throughout the land as not fixing issues. And any underpaid call centre operator in Manila who tells you differently is reading from the company line.