I actually don't know that much about it, but... I'm there ;)
It seems to me that France is rather "dull" as far as looks go, compared to UK or Germany.
I know there are quite a couple of goth or metal-oriented shops in Paris for clothes and accessories (most of them in the same neighborhood, I'll list the names and addies for you if you want), and lately it has spread enough that you'll probably find at least one in each town relatively big.
But people I see around that area don't wear hair falls like the ones you do. Mostly just the regular black hair, or dyed. Then of course I mostly see them during the day, so maybe they only get out the real ornamented stuff for parties.
In a way it means you have an open market, but to get the extensions, it might prove difficult if nobody else uses them.
When I looked for a wig for my Death costume, I had to resign myself to buy a rather bad one in a costume shop. Fortunately, I went in London a month later and found there, for cheaper, a much prettier wig, better styled, and more realistic (the French one looks plastic from 10 meters away...)
So... a BIG market ;)
You know, I was actually starting to think that since it's not that widespread yet, you might even consider striking deals with those goth shops I mentioned. I know they already sell stuff that's made by artists / craftsmen, not just established companies.
It might be more complicated legally and they'd get a percentage, but you could probably a kind of business plan to make your falls known in the little word of French metal faster :)
So the plan is : go out and show off a LOT in the first months ;)
For fees, if you find a website located in the European Community, you'll pay no fees at the customs.
If it is outside, I think they'll apply the VAT (18,6% in this case).
It may depend on the value of the package, because I was once told that under around 500 Francs (75 euros), the cost of making you pay the VAT exceeds what they'd recover on it (time spent treating the case, paper work etc).
That's why I never had to pay any extra fee on the few things I booked from Amazon US or other US companies, because I never order a lot at the same time. But it's a tip, not a rule.
And maybe they'd be more suspicious if you order material that is only used by professionals in general.