bondeziro

Two roots you already have, welded into the word you need at the end of every letter. Bona plus deziro, the noun from deziri, gives a good wish. Main root last as always, so the thing is a kind of wish and bon- says which kind. English has no single word for it and has to say best wishes or good wishes; German gets there in one with Glückwunsch, and Esperanto sits with German.

In practice it is nearly always plural. You send bondezirojn, not a single bondeziron, in the same way English sends regards rather than a regard. The standard letter endings are kun koraj bondeziroj and plej korajn bondezirojn, and the verb that goes with it is esprimi bondezirojn or simply sendi them. The related verb bondeziri also exists and takes the person in the dative sense with al: mi bondeziras al vi.

You will meet it first at a birthday. The festivals article has the awkward truth that there is no standard Esperanto Happy Birthday song, and that among the several texts clubs sing to the borrowed tune, one is simply Bondezirojn al vi. So the word turns up as an entire song lyric before it turns up as vocabulary, which is a good reason to know it.

Its opposite exists and is worth knowing. Malbondeziro is barely used, but malbeni, to curse, is the word for the real opposite act, and bondeziri sits against it neatly. There is also bonvolo, goodwill, which is a disposition rather than a thing you send.