http://5rzkisd745el4yr7krh5qc4f5sgblvg4i5qtuxivfgxcxtr65ejd7gyd.onion/help
Per message thread: the ciphertext of every message in both directions, the public key you supplied so replies can be encrypted to you, and timestamps. Only the hash of your thread secret is kept, never the secret itself. Note what this means in practice — the shop is holding a conversation it cannot read one side of : your messages need the operator's private key, and the operator's replies need yours, and neither key is on this server.