Yes, it should be "Suratnya" or "Surat dia". Both mean the same, meaning "His / her letter...".

In Indonesian the "Item" comes before the "person". "Bola saya" - My ball "Ayahku" - My father "Bukumu" - Your book "Rumah Anda" - Your house "Makanan kalian" - Your (plural) food

Hope this helps!

I think it is informal. Because tahu means both know and tofu and my indonesian friend says to distinguish it, they say tofu is 'tau' as slang