Buyer wanting different rate to pre-paid label

I've just had my first sales with the Simple Delivery pre-paid labels, and run into an issue. A slightly awkward buyer (who has bombarded me with questions for days) has purchased an item with a Tracked 48 pre-paid label. He has now asked me to send the item straight away so it goes 24 hour delivery. Given that he's selected a 48 hour label which doesn't necessarily have to go next day, how is he expecting me to do this? Is there a work around either of us can do, or is that option set in stone? I'm still getting my head round it.