Terrible customer service

I bought some Nando's vouchers on the AVO site. I ordered and used the vouchers, but didn't notice that even though the vouchers were captured that all of then were not taking into account when checked out as the amount that was charged to my card differed from the amount shown on the screen. I have mailed the customer service line on 9/11/2023 and didn't get a response. I then phoned them and didn't get far as i had no proof as I don't suppose any one record a video of their online transactions. The person asked me to send the voucher numbers to check if they are still valid. I then mailed them on 12/6/24 again and got NO response. Really disgusting service. I just want my money back.