BAD SERVICE

I went to BP filling station to get petrol using my vodabucks voucher. I first asked the petrol attendant if I can use a voucher and she agreed, I then purchased a voucher and when she was done filing the petrol she told me that she made a mistake she doesn't allow vouchers. I had to pay again from my pocket, I alerted the manager and sent numerous emails to BP SA its been 3 months they are not even communicating with me. Yesterday I went to a different BP tat is located 200 km from where I stay and still I couldn't use the voucher , I was told that the person who knows how to help me with the voucher is not at work.