parcel was returned by French Custom??

The receiver address may be wrong.

I know many cases in Belgium and Germany, none from France !

I never had problems receiving one parcel from abroad (and I received hundreds of them).
You should write "Dried butterflies specimens for scientific study. No commercial value. No protected species".

About live insect, it could be illegal if the species is listed as a potential pest.

I thought that I was the only one that had problems sending boxes of dead, dried specimens from the U.S. to Europe. In the last two years, I have sent parcels by Registered mail to France, Spain and Germany. Most of them arrive fine in two or three weeks. However, I have had three sent to France returned to me, two to Spain and two to Germany also returned. NO explanation was given at all. All the boxes had full customs declarations , and were correctly addressed. A few boxes of specimens sent to Italy never arrived and the American Post Office could never trace them. The most annoying thing is that it takes on average six to 15 months for a registered parcel to get back to the US. sit in Customs forever and finally get back to me !

In my case, I never had any problems at customs.

By cons, and it happens more often, I do not receive some parcels addressed to me at my home in Paris. The main reason is the following (I inquired at the Post Office!): More and more often, the delivery of packages (unlike letters) is provided by private companies (and not, as c 'once were, by the public service of the Post). But

a) the deliverymen from private companies do not know the access codes for houses and apartments. So they can not deliver the package or even filed a notice of passage. Unlike postal public service who have the codes or have passes.

b) These deliverymen private companies are subject to performance standards that lead them to make their journey as quickly as possible. Arrived before a closed door which they do not know the access code, they leave without even leaving this calling card.

It is a practical effect of the policy of destruction of public service conducted in France by the last government. Same as the results of the Thatcher's policy on english railways !

My advice is as follows for those who want to ship a package to a French resident in a large city (in the countryside, this is yet another reality) to indicate the destination address with the code (s) gateways and enter the mobile number of that recipient. And, for the driver to contact the recipient without an excuse to leave with the package!