Update on the kitchen

So, when a customer orders two worktops in store, pays for them, pays for their delivery on a specific day (in 4 days time), when's the best time to tell the customer that one of the items isn't in stock? Yes, that's right 40 minutes before you deliver one of the two worktops. 

I'm so annoyed by Ikea over this. I mean, when you order the worktops it's done from a computer in the store so it's not like they can't tell if it's not there. Also I could have wanted to pick them up that day. Was I supposed to get down the road to the pick up place only to discover one of them wasn't in stock? The thing is I used to work on stock control software so it's not like it's some black art to know what you have in the sodding warehouse and be able to tell the person ordering the thing. Or did they have one in stock that day and then sell it to someone else in the mean time?

*fumes*