I don't know about that!!! The whole emphasis about washing your hands is that you can pick it up by touching contaminated surfaces and then touch your face with your contaminated hands. The virus gets on the surfaces either because they have landed on them from an infected person's mouth, or because somebody has coughed onto their hands and then touched something and contaminated it. It doesn't die straight away, they say that it can survive on plastic or glass for a week, maybe under certain conditions, paper for maybe several hours etc. If it lands on a dry towel it doesn't last long as the protein spikes get pulled off as the water is absorbed. Re letters, an actual letter is okay because any viruses on the paper will have died by the time that you open the letter. But I would be careful about the envelope - pick it up, open it, throw it away straight away and then wash hands making sure that you don't contaminate anything, especially your face, in the meantime.
There was a "How Clean Is Your House" a few weeks ago on Channel 4 which was very informative (don't know if you can get 4 On Demand in Germany but worth watching if you can), also an American Department of Health or equivalent film on Youtube where there is a woman who does everything right in a supermarket but touches her phone with her gloved hand by mistake. The Virus is simulated by a purple powder and it shows how she then uses the phone in the car park after she has finished shopping, gets the virus on her hand, and then contaminates her car door handle, steering wheel, etc.