In the UK, Royal Mail offer several "signed for" services.
The simplest is "Recorded Delivery".
A user takes their item of post to a Post Office, and pays an additional fee to the standard postage. They receive a uniquely numbered receipt.
When the item of mail is delivered, it must be signed
for by an actual person to acknowledge receipt. The name and signature are recorded, along with the date of delivery, and are accessible online and (if required) as treeware. Such a record is legally evidential.
The necessity exists to submit an item to the system though a Post Office, which have limited opening hours. This can be somewhat inconvenient.
But the essence of the process is that
(i) the carrier should collect the correct revenue, and
(ii) the delivery of the item is recorded in an evidential record.
Neither of these two processes require the item to be submitted via a Post Office.
Adhesive labels are available free-issue upon request in all locations which vend postage stamps, similar to Airmail/Par Avion labels.
The sender must also make sure that the item bears their own address and Zip/Postcode, and those of the destination address.
A service user may optionally affix such a label to their item. It is their responsibility to ensure that the value of postage on the item is sufficient to cover the relevant charges. Should the postage be insufficient for the recording service, the item is delivered unrecorded. Should the postage be insufficient for the size and/or mass of the item, the recipient is surcharged in the usual way.
If the postage is sufficient, on delivery to the stated address a signature and name is obtained and permanently recorded
By entering the Source and Destination zip/postcodes and the date of posting into an online portal, indication of delivery is displayed. Should evidential documentation be required, a further fee will produce a certified treeware record.
This scheme would only be suitable for items of little or no intrinsic value.
The onus is upon the sender to ensure that the appropriate fee is affixed to the item.