You don't even need to explain why. Just say "I need the computer that comes in the smallest box. I know this is weird, right? I need it for reasons that would

You don't even need to explain why. Just say "I need the computer that comes in the smallest box. I know this is weird, right? I need it for reasons that would take a long time to explain, but I promise they make sense." A bit of empathy for the baffled Apple store employee on the other side of the conversation and nobody has to waste time or feel stupid. Maybe there's additional context we're missing.