Well, today I received an empty post bag. I bought a small pendant, so it was sent in a small padded mailer, as registered post. That was still stuck fast when it arrived, and I definitely opened an empty bag. What do I do? I feel pretty sick to be honest. Have contacted the seller, no word back. I doubt I'll hear back.

Yes Dee, that is correct, you can only open a dispute when the final ETA has passed.