"Tini sends a letter to Andi."
Translation:Tini mengirim surat kepada Andi.
Hai. Kepada is apparently used when someone or something (a letter in this case) is going to a person, as opposed to a place.
I'm a Malay speaker, but afaik
kepada(means to) specially is used when the stuff to be/being delivered will be sent to a living thing (like a letter, ball)
ke(means to) can be used (at least informally) in both of the cases (the moving/thing being delivered will be sent/arrived at something living or not living)