"They do not write letters usually."
Translation:Elas não escrevem cartas normalmente.
normalmente can go either at the end or beginning of a sentence. but it's hard to settle a rule once it may vary on the intonation, region, or other factors. geralmente eles vão lá = eles geralmente vão lá = eles vão geralmente lá = eles vão lá geralmente. use it before the verb and you will always sound natural ;)
A question about word order. In English, it is considered best to have an adverb as close as possible to the word it's modifying. By putting "usually" at the end of the sentence, it makes it look like they mean: "They write letters in an unusual manner." But what I think they intend to mean is "Writing letters is an unusual thing for them to do."