It is a fine idea ! The only problem is the time of transit for your cutting: first, Honolulu---some city in the US (wich can take more than one flight I suppose), reception of the package by your friend, repackaging then sending to Germany (are the flights directs between USA and Germany ?)...I think that a Piper m. could survive one week in a box without light but more than that it could be difficult to revive, add to that the stress of the multiple chocks the package receive, the temperatures variations, eventual customs check and rough handling...
Maybe it would be better to ask for an american friend to receive the plants, cultivate the plants for a while and let the plants relax before another trip.
In my opinion, the best option is unrooted cuts in moist sphagnum: it travels well, doesn't really needs light, stay humid, don't care of the chocks since there are no roots nor leaves and it can even start the rooting process during the trip ! I've had some success with tropical cuttings before, even with a two weeks long postage.
Usualy i receive kava (powder) in 5 days (I am living in Okinawa) so I am hopeful that maybe one day...
The ebay vendor send only to US territories...the funny thing is that Okinawa was an american territory not so long before !