Faulty Nandos App resulting in double payment

I placed an order online on the Nandos app and I had a discount voucher which I punched in them pay the difference on the same app. When I went to collect my order I was told that my order was not paid for. I was shocked because I got a message on my phone that a Nandos payment went through. Then the cashier told me that it was R25 short which was the same amount of the discount voucher I had. I asked how can I make a payment that is not full amount because you don’t get that option on the App, it’s either you pay the full amount or pay when you collect. Luckily I had my card with me, so I paid the R25 to release my food. Otherwise it was going to be a fruitless drive to store.