Possible Clues:

• Two things Charles is but Camilla isn't, in speech to deliver abroad?
• Fast delivery
• Trouble behind goal with header finally going in -- it flies in!
• Display armour in overseas post?
• Post taken up by qualified pilots?
• To be sick confines girl to such correspondence
• Excellent marines upset getting post in the sky?
• Post sent overseas
• Overseas postage
• Post sent by plane