Mustve done, i sent a 14XX back a few years ago and im pretty certain that was a prepaid collect plus label.
They did say that they would cover basic 2nd class post - basically the cheapest option. I asked for a prepaid label mainly because if it goes missing its thier problem as they are the ones with the contract with RM. If i post its me with the problem as the sender. They did say dont worry just get a proof of postage - but thats very basic and it lacks the type of item handed over - ie parcel, letter, large letter etc and crucially it dosent have the wieght on it.