"I like your letter."
Translation:Saya suka surat kamu.
Yes, it would be as Saya is the formal of Aku.
Whenever I've learned Indonesian in a formal setting, they would always teach us the formal sentence structures rather than informal because you would run the risk of insulting someone unintentionally if you spoke informally.
The answer is "Saya suka surat Anda" - is "Anda" usually capitalised?