Was the message direct via the eBay message system, or a note added to the 'request invoice' function? Not that it makes a huge difference to the current situation as it stands, it's just that messages added when you use the 'request invoice' function are super easy to miss. They're tacked on to the very end of what is a fairly messy format, and in relatively small text, plus there's nothing to indicate special requests have been made (admittedly most buyers will message direct to make special requests for things not offered in the listing, but I've learnt to double-check invoice request notifications, because sometimes people do it that way, and because it comes through much like all of eBay's other automated and superfluous emails, occasionally even going to the spam folder).
Still, I'm not suggesting it's your fault if the seller missed the request, just advising that - on the off-chance it wasn't via direct messaging - that would be the way to go in the future to ensure the seller is aware of your needs (preferably before you even buy) and that they can accomodate them.
You can't cancel the order, as such, but if the seller accepts returns, it might be best to try and arrange for the item to be sent back in your absence.
