FWIW, what I suggest is exchange addresses online in personal message, but do so while the other person is ready, and delete it immediately once they confirm that they have it. As long as it isn't left there, risk is low.
Corrugated plastic sign material is just thick enough to hold seeds, but still thin enough to be included in a greeting card. I know of seeds in envelopes crushed by mail equipment in the USA, but not when sent in the sign material. The contents of letter envelopes or card envelopes does not need to be stated, and at least in the USA, ordinary letter mail can't be opened for inspection (or so I have been told by an American who seemed to know).