This case has been escalated to our transportation team. It seems like there were some miscommunication within ThaiPost, this issue should be corrected in a few days.
Please find below the reply from K. Narong who is looking after transportation:
"It turns out that the post office misunderstood about the return process for LZD. So the head of post office already re-communicate the right process to the officers again.
We (transportation) will contact the head quarter to make sure that every post office can facilitate the merchant for returns.