The Swiss Post currently has a stamp-by-SMS service for normal letters where upon sending an SMS to an automated system, you are charged the normal stamp price

The Swiss Post currently has a stamp-by-SMS service for normal letters where upon sending an SMS to an automated system, you are charged the normal stamp price using the premium SMS system, and you receive a sequence (8-10 characters long IIRC) which you write where a stamp would normally go at the top-right of the envelope. It's far more convenient than having to remember to buy stamps in advance or typically going out of your way to a post office just to mail a letter as mail boxes, by contrast, exist seemingly all over the place.