Usually do not have problems with customs and transponders.
I have many clients within Europe, including Germany, Netherlands, Austria, France, Latvia, Greece, Spain, Denmark, Sweden, Norway, Belgium, Georgia.
The important thing is to value the shipment to me in Australia at under 600Euro to avoid customs import duty to Australia.
When I return them I include the freight documentation that shows it came from the country it is being returned to and a low value commercial invoice.
Freight companies can have issues with transporting items by air which contain batteries. Batteries sealed inside transponders are safe to travel by air.
Provided they are packed securely so they cannot be damaged in transit.
Some mis-undertanding can occur because of issues around Lithium batteries. So if battery is mentioned to freight company they only think of Lithium danger. Most transponders contain Ni-mh or Ni-cd so are very safe.
Matthias, if you contact me directly via email link on my web site and tell me which type of transponders you have I can advise you further.
Transponder Services www.transponderservices.com