Force yourself to smile on the phone, it does go through the phone wires, and apologize. No need to take responsibility for the mistake, but sincerely say you are sorry and you would be happy to make it right in the customer’s mind. Offer to re-make the pizza and send it out free of charge, and let them eat the one they have while they wait, and throw in a 6-pack of cokes because they had to wait an extra 30 minutes before eating dinner.
What have you got to lose? $5-6? What is the cost to gain a new customer? What is their potential cost over the next 5 years?
You have a POS, use it, and make sure that these guys never get the topping they are allergic to again. Set a pop up message, and you will be fine for the future. Give them the benefit of the doubt, and bend over backward. They will be impressed.
Yes, it is a given that any fool is smart enough to mention things they are allergic to, and these idiots didn’t, but why piss them off in the process. Take the high road, and do what Wally did. (Then make sure they don’t have the opportunity to do it again).