Had to pay twice- they don’t recognize the voucher when I ordered on-line

I ordered on-line as per order: [ I can confirm the order no]. The total order was R300. I paid on-line R150 with a voucher: [ I can confirm the voucher no]. I then paid the rest of R150 with my credit card on-line. When I collected the order, they didn’t recognize the voucher and said I had to pay R150 again. I then paid the R150 again even though I used the voucher because they said so. So I effectively paid R150 twice. When I complained at Nando’s, they kept saying they cannot see that I paid R150 with the voucher, yet it reflected when I made the on-line transaction. I gave Nando’s the voucher & order no’s. Until now I haven’t received any further feedback. Surely Nando’s should be able to trace the transaction and confirm the voucher & order? It’s not nice paying twice...