Possible Clues:

• Speedy deliveries by road and train covering miles
• I'm a liar (anag.)
• A sweet French girl I left, being sent overseas
• USPS option
• Post via plane
• Overseas post option
• Such a post makes top-class marines feel ill
• Letters by plane
• Let 'er be flying, by the sound of it
• The letter shows he inherits, perhaps, by the sound of it