Stupid people making me look bad

This woman came through my line and started putting all her groceries on the cart like normal. I said, "Hi how are you?" and she smiled and said, "Good thanks." I started ringing her items through which she can clearly see and not to mention the belt is moving. My bagger bags the items and puts them in her cart when she brings it down. I then tell her the amount and she hands me a piece of paper that makes me want to rip her throat out. "Oh, this is a WIC," she says.
When someone has a WIC they are supposed to tell you before you start because on the screen you have to hit "WIC transaction" and type in all the dates, and put the voucher through the printer. It's a process. I tried really hard to keep my face calm, but I'm not sure how well that worked out.
I flashed my light for asisstance and Christina came over. What I wanted to say was, "This idiot forgot to tell me her order was a WIC so I have to start over." But the customer is always right so I said, "This was supposed to be a WIC." Christina sighed and started to void off every item one by one and voided out the order. Then we had to take all the bags out of the carriage and take all the items out of the bags. I stood there uselessly as Christina hit WIC transaction and rang through every item again.
At the end she handed me the receipt to give to the customer, glared at me and said with such an attitude, "Next time hit WIC transaction." She walked away in a huff and I was left standing there with steam coming out of my ears. I handed the receipt to that insufferable woman and didn't say a word.