"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.
Same question here. What’s the difference between ke and kepada here? Terima kasih
Maybe because the noun or group of nouns in front of mengirim becomes the thing being sent. So if youre sending (mengirim Andi surat) then you're sending the Andi of the letter(s)...which sounds very weird.