There used to be a premium number you could send an SMS to. It was more expensive than a normal stamp to cover for the expensive billing. "Handyporto" replaced

There used to be a premium number you could send an SMS to. It was more expensive than a normal stamp to cover for the expensive billing. "Handyporto" replaced this and that's probably also why you can't buy it on the website. Deutsche Post probably can't conceive of people who don't own a printer so they heavily advertise "Internetmarke" which requires one.