Let's focus this story solely at a pickup location, the terminus of all transactions. Whether it's the pharmacy or a pizza place, once something is ready, you cannot change the order without consequences. In this case, the result is a longer wait. You have altered the terms of our interaction. Sometimes I feel like Lando facing Vader. "I am altering the deal. Pray I don't alter it any further."
Picking Up Something-Extra: I am here to pick up my pizza.
CP: Got it. I have 2 large pepperonis, an order of BBQ Wings, and a 2-litre. Is that correct?
PUS-E: Yes. Yes it is. It's exactly what I ordered.
CP: Okay. The total is $22.49.
PUS-E: What about the garlic bread?
CP: What garlic bread?
PUS-E: I was supposed to have garlic bread. It's part of your meal deal coupon. I have it here.
CP: Did you tell us you had a coupon for a meal deal?
CP: Did you ask for the meal deal?
CP: Did you ask for garlic bread?
CP: When I read you the order mere seconds ago, and you said "exactly", were you being facetious or just dim?
PUS-E: Huh? I want garlic bread and to use my coupon.
CP: Okay. We will fire up some garlic bread for you. It'll be another 15 minutes to get it ready.
PUS-E: But the game starts in 5 minutes.
CP: I know. I was hoping to listen to it on the radio. We've had orders coming in all night. Just ask all the people behind you who also wish to make it home in time for kickoff.
PUS-E: Your machine told me my order would be ready at 7:45. It's 7:47 and now I have to wait another 15 minutes? This is ridiculous.
CP: It was ready at 7:45. You've been arguing with me for 2 minutes trying to change your order. You are welcome to take your already-completed order, minus the bread of garlic, and hie thee home, tarrying not.
PUS-E: What about my breaded garlic?
CP: Your original order, as originally placed, is complete and ready, in its entirety, for you to take home and slam down your gullet whilst cheering on your favourite club. This transaction could have been completed minutes ago had you not altered the terms of our deal.
PUS-E: Well how much longer now?
CP: It's still 15 minutes.
PUS-E: What? Why?
CP: You've been arguing with me for 5 minutes now. I've had no time to walk away and place your order; an order you've yet to assent to me making.
PUS-E: I'll wait for it. But it's all your fault if I miss kickoff and if my food is cold and...
CP: Yeah yeah yeah. I get it. Next!