I would be a bit wary of that and probably (politely) refuse, although it would depend on how I was originally going to send the item and what the value of it was (i.e. if I wasn't going to have PP protection and it was only small amount of value).
Although, in saying that, if they're the ones paying the bill for the reply paid service they'll have to pay P&H costs one way or another, and if they're not, it seems like it might not be entirely on the level.
