In Germany we have the “(Übergabe-)Einschreiben” type of delivery, which requires you to give the mail to a postal worker directly, who will then give you a receipt. Then the recipient also either has to sign for it or pick it up at the post office it got delivered to. This pretty much guarantees that your letter arrives properly, but I think there is also an actually insured tier of service, but that requires telling the contents to the postal worker in detail.