I thought that they'd given up, but I just had another email. This one telling me that ebay has advised them to contact me to sort it out - and as usual my reply is that they have to go through ebay official channels and actually open a case, so that I can report it to the post office and police. For some reason they seem to be very reluctant to do that.

At this stage I really don't care much - I can prove I posted the goods, although not what was in the package, and that they received it, and that should be enough to get ebay seller protection. I just find it all a bit annoying, especially the insults to my honesty and (if I'm right to think this is an attempted con) my intelligence.