Buy two get two coupon

As we were leaving the house, I checked my mail and received a Carl's Jr. coupon for their new phillysteak burger. Ok, lets try this sandwich. I knew that going through the drive-thru was mistake but went anyways. I clearly state into the little order box, "I have a buy one get one free coupon for the Philly Steak Burger. " The lady says, "ok. That's $3.76."

Not that simple. I get to the window and pay and the guy hands me the bag. I check to make sure this monsterous order is correct. Nope. Just one sandwich. Remember, I'm in the drive-thru!

Me: I'm missing my free sandwich.
Guy: You paid for one sandwich.
Me: When I ordered at the box, I stated I had the coupon.
Guy: We don't do that. You have to buy another sandwich because you didn't give me the coupon when you ordered.
Me: I was at the box, I ordered it with the lady. How was I supposed to give it to her?
Guy: Next time you can get your sandwich.
Me: No ... I'd like my free sandwich, please.
Guy: Well, I didn't press the button for the order to say free.
Me: Of course not, you didn't take the order. She did.
Guy: Guy hands coupon to lady that took order.
Lady: Ok, I'll give you the sandwich free this time since it was my fault. She tells the back to make another sandwich.

I think this time, huh. You're doing me a favor. I park the car and the lady brings it out to me. I thank her with a smile and drive away.