"Dia memberikan saya surat."

Translation: She gives me letters.

[Roy803156]

Sometimes translation is confusing. Local people don't use a classfier if it's one letter

[Chuck919019]

Weak distinction betw Letters & The Letters...

[JugderGurr1]

If she brings me one letter,would it be sehelai surat?

[Rick392366]

You can use "sepucuk" as a classifier.
You can also use the generic "sebuah" or use numbers if you want to be very specific about the quantity :
Dia memberikan saya satu/dua/tiga surat.

Here is a topic about the classifiers :
https://forum.duolingo.com/comment/3842014