account billing

i am so disappointed right now with your consultants. i placed an order , then they included this 100 off voucher in my order and deduct it from the balance. but after paying the total amount (balance)that appears in my account, i found that my money is R150.00 short because that voucher is for those who buy on credit .why did they deduct it knowing that cash customers do not qualify. the 100 off and 50 off shows in my account that it was deducted because the total amount appears which i paid based on the statement