I'm sorry to answer so late, but hope that I can contribute to this interesting subject.
Of course, I noticed, when I receive your seeds, that usually, the fare is higher than the french one.
I made an investigation and found out that the law in France is interpreted in different ways by the post-office employees.
No special restrictions affecting seeds -
Requires that the letter is not more than 2 cm thick (entry of a standard letter box) and there is no obvious risk of damage to the passage of automatic sorting machines. The regulation is even more "wide" than that.
So, I generally use the special machine, when I go to the post office, for weighing my envelopes (bubble ones to protect the seeds) and I'm not sure the bubble envelopes are necessary, because a seed is very hard). I just click on "mail" and not on "goods" (marchandise) and this make a big difference. I'm doing this by myself and put the envelopes in the mail box and don't have to queue and no talking with the employees.
Regarding all this, I can advice to the european donators and if they can use the same weighing machines, assuming the laws are identical within Europe, to send their seeds in small quantities (it is much better for our seed list to propose fresh seeds every year) every year. I don't need lots of seeds, especially if they are tiny ones like Papaver or Nicotiana ones. I can remember somebody sent me perhaps one hundred pods of Thevetia peruviana. First of all, I would have prefer having the seeds cleaned of the pods, which are tough and heavy. I suppose this had a very high cost and it could have been avoided. Donators should avoid too, sending very big seeds as palm trees ones (wider than 1 cm width), because, in that case, the envelope would be 2cm thick.
The price in France for sending normal mail in Europe but over 20 g, which occurs regularly when I send seed orders, is 1,35 euro. Under 20 g, It is 0,95 euro.