You can also send letters insured, for example, if you want to send money. This is called registered mail.
There are four ways to send a registered mail:
Certified Register mail (extra charge 2,35 €):
The deliverer documents that the shipment (letter or package) was dropped in the mailbox of the recipient
Registered mail  (extra charge 2,65€):
The deliverer hands over the parcel personally against signature by the recipient or a representative (eg spouse)
Registered by hand (extra charge 4,85€):
The deliverer delivers the shipment only personally against signature by the recipient. Should someone else accept the letter/package he or she has to present an official authorization paper (power of attorney).
Registered letter with recorded delivery (extra charge 4,85€):
The deliverer hands over the parcel personally against signature by the recipient or a representative (eg, spouse), and they get the acknowledgment of receipt sent to the sender.