I used to be a postie, things may have changed a bit, and it will likely depend which country you are in, however....
Interestingly, sending a letter without payment typically used to have a better chance of ending up at it's destination than if you include payment, it's just that it costs the receiver, plus iirc there is an admin feed sometimes for them (if they choose to receive it).
The curious thing here, is that because the letter gets handled differently, and needs payment, it's almost like a registered letter, i.e the postie will knock on the door and deliver it and take payment, or will deliver a card for you to go and pick it up from the depot, where they will pay (rather than getting dumped with a load of other mail which could end up in the neighbours house). Naturally the recipient may not want to pay for it, so then it will go back to sender if there's an address, or black hole otherwise.
I wouldn't test this out with anything important though :).