I have done it several times myself as well as received many packages from dealers done this way. This is an example of a great way to use 1978 12c postage up. I believe as long as the value is showing and the country is shown it should not be a problem. As for the reuse of postage, that is a definite no-no. It could easily be detected by the bean counters and if they catch you it definetly puts you in a position for a "big hurt"

as one Canadian comedian says.