Lancome complaint: Worst customer service providing incorrect information leading to losing a $40 voucher

I made an order three months back using a $40 voucher and then realised a I made a mistake. I contacted them on the phone and was advised that I can return the order when I receive it and that I will be redeemed the voucher. I made a new order (the correct one) and had to wait for the first one to arrive and them instantly I posted it back. Processing the return took a time longer than advised and I had to chase them to process it. After it was processed I was not redeemed the voucher and when calling them they said it has expired. No one had ever mentioned that it will expire after 30 days from generation. It was their fault that the returned order took too long to process plus I have never received any emails as mentioned on their site saying that the voucher will expire 7 days in advance. On calling them today, I spoke with the manager who did not provide any solution except that they are sorry. No voucher no offers nothing to compensate for my loss of $40 due to their mistake. Worst customer service experience ever