Rude manager and no customer service

Yesterday we had gone into the Adega at gateway to use our 'Groupon voucher. When I got to the restaurant I was told it was school holidays and that I could not use my voucher I hadn't realized that and so I thanked the gentlemen (waiter) and was about to leave when he stopped me saying 'But we can allow you to use the voucher today if you like...And you would have to pay a fee of R17.10 I responded that I accept and would pay the fee. As I began to sit down at our table the waiter came back to me and said 'sorry the fee is R40 not R17.10 I then asked him How come? And he said speak to the manager. The manager Johnathan came over to our table and was extremely abrupt and rude, without asking me anything he said the fee is R40 and you should have read your voucher'. I wasn't even given a chance to tell him that the waiter told me R17.10, after Johnathan was done I then said to him please help me understand the R40 as I would have been happy to have left the restaurant if I wasn't allowed to use the voucher but I did not understand the different fees, he mumbled something about a 10% gratuity. I said that it was ok I would pay the R40, I was already seated and embarrassed because o