Account charged for delivery after checkout

I received a R100 birthday voucher to use online so I ordered last week, my final total to pay was R257.80, this was after the voucher was used and the billing included free delivery because the items purchased were over R350. My order was delivered now and I see a R65 delivery was charged. How was my account charged after I qualified for free delivery and only checked out because of this amount?? I made sure my billing included free delivery after I checked out. CNA, this is why your sales are so poor, incorrect billing, and charging the customer after the fact. Never again..