I'm in a similar position. I'm wondering what happens if you simply fail to pick something up. The in-store pickup process gives quite tight 15 minute timeslots, at least that's how it is in the UK. In my experience they are pretty flexible if you are a bit early or a bit late although I've only ever tested that by being early on a few occasions. I do wonder though what Apple does if you haven't picked up your order at all by close of business on the day of your timeslot.
I presume Apple doesn't patiently keep the product you ordered/reserved set aside for you indefinitely so at some point something must be done to release that product back into the new stock. Maybe initially Apple sends a "you missed your pickup slot, do you want to reschedule?" type of email and maybe at that point you have the chance to cancel. If it does that I would think that it would be generated automatically during some end-of-day processing after all the stores have closed in whatever country the person is in. Alternatively, perhaps Apple is more brutal and goes straight to a "you missed your pickup, your order is cancelled. Place a new order if you still want to buy this product" type of message.
Someone here must have missed pickup completely and know what happens.