I have sent seeds in bubbled envelope, even larger quantities, not like 300g as your link, but still 2 dozen or maybe more varieties, each of them with minimum 10 seeds, some with 100, also small pieces of dried peppers, and never had problems. All of them arrived nice and safe. I have sent them from local postal office, not through customs. Nobody ask what's inside. Just weight the envelope and put the stamps according to the weight and distance. I get tracking number for them, and can track till leave the territory of Romania, and I found the tracking number later on the USPS site when enter the US territory, can see also all the operations on the US territory.
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I have sent even fresh peppers just to Slovenia, in a bubbled envelope, no tracking number, simple sending, and guess what, paid only about 0.4 euro!