Is there postal tracking for letters?

With the new Money back guarantee policy thats coming in, i have concerns. If someone buys an item, then receives it, then decides to pull a swifty to get their money back. they just say it didn't arrive. It is then in the sellers hands to prove that 1 - they posted it and 2- the buyer received it.

Now while this is going on sellers will get a deficit, they will also have their paypal accounts frozen. The problem i have with this is that i sell a lot of things that get posted as large letters. While i get tracking on parcels to prove postage and delivery, this option does not apply to letters, as far as i know. How am i to prove that i have posted the letter and even more impossible to prove, that the buyer has received it.?

maybe i will have to put all my prices up and box my little letter items so they can all go as parcels with tracking.

Any other ideas please