Does ordering online not guarantee your order?

Idk if I'm being a Karen or just pregnant or both but either way.

My store's mystery is oreo mallow sandwich, which I was excited to try. I place an order in the morning for a pickup time after work, since I know the cookies often sell out during the day and I figured the mallow sandwich would be popular. Got a 4 pack of 2 churro, peach (husband's request), and mallow.

I get there and it says sold out of mallow on the screen, and when I tell them I'm here to pick up they ask what I'd like instead of mallow since they're out. I asked if ordering and paying on the app ahead doesn't guarantee my cookies, and they said no, they make the cookies to order so if they're sold out, they're sold out.

I thought that having a prepaid order would make sure they set one aside? Especially since it's chilled? Has anyone else run into this? For the record I didn't go full Karen or chew them out or anything, I'd just like to know if this is standard practice for future reference.

Edit: I appreciate all the feedback! What I've learned is there's absolutely no point in ordering on the app so I'm going to go ahead and delete it