We decided in the end to pay the return documents fee. We sent out the package to NZ about 2 weeks and we've just received this morning our original documents back again via DHL.
They are now verifying the qualifications so we are just hoping there are no problems. But anyway it seems that paying the return documents fee is a fairly safe and fast way to do it. It was certainly easier and cheaper for us to do it that way rather than trying to find a courier bag.
Just thought to let people know our experience of it.
Cheers
Starkhorn