It's often much better to post seeds in a letter rather than in a package. Depending on the country, what is considered a "letter" might be quite big (up to 5mm thick in Belgium which is often enough, but up to 3cm thick in France) and regulation is completely different with usually total privacy regarding the contents of an envelope, compared to a package where contents must be declared.
So for seeds, in Belgium or France I will use an envelope with some padding inside, buy the stamps separately and post the envelope in a mailbox. No questions asked this way, and it's also much less likely to be opened by the customs in the destination country. And usually much cheaper as well.