Estonia could have fit to the campaign too (Boletus in Estonian is "puravik"), but our sitting government has different priorities.
I imagine the three Baltic states have much in common, so being represented as one seems to make sense. Has that been done in the past?
Actually Estonian language is very different from Latvian or Lithuanian (they are pretty close to each other). Estonian "puravik" is one of the most similar words with the analogues ("baravika" or "baravyka") in Baltic languages. Probably, because of its common Russian origin ("боровик"). In Finnish language (that is very close to Estonian) boletuses are called "tatti", in Estonian we call "tatik" or "tatikas" only members of family of Suilluses (Suillaceae, also under Boletales). My guess is, that ancient Estonians called all Boletales as "tatikas", but later Russian influence caused different naming for Boletaceae. Of course we do some regional cooperation with Baltic states, especially in environmental and defence fields, but in general our relations are pretty weak. AFAIK, Estonia has had its own pavilion on every EXPO, where Estonia has participated and spent ca 4 millions of Euros each time. For upcoming EXPO our government decided not to participate (the only member state of EU, that won't participate in Osaka), mostly because of economic reasons. (https://news.err.ee/1609001762/government-confirms-estonia-will-not-participate-in-expo-2025). Estonia was looking for 300 m2 individual pavilion, but cost for such rent was too high and cooperation with Latvia and Lithuania was also rejected (for not disclosed reasons).