From my reading Kordon recommends fish such as "gouramis"(labarynth) fish be not shipped in full bag,but rather "less then full and then double bagged".
Lots of controversy on double bagging breathers but kordon claims only a 50% loss a breathability in double bagging.
if the bag can't be full then the breather is not needed.
The full bag is so fish don't get thrown all over.With air in bag this is just a fact and not a lot anyone can do.
We have shipped fish old school along time pretty well?(popular hobby)
Most shipping issues arise due to shipper/carrier not giving a living !@#$.
I keep good relations with the owners of my LFS
(top2 at least),
and you should hear the horror stories of lost shipments , boxes sitting on smoking hot runways for hours!
They both even mentioned the same airport as the worst!
That being said after vetting just tell your guest to transport respectfully and use regular bags or a Tupperware type container.
I have rambled!