It isn't necessary to specify singular or plural unless there is a specific context. I'm not too sure of the exact context/situation which you may be referring to, so I provided context in my response. If it's just a general suggestion...you can probably say something like: "You can always send a letter to her address if you ever need to reach out to her." This would imply...that you can send ONE or MORE letters to the address...as many as needed/required. Similarly: "You can always send letters (plural) to her address..." ...does not necessarily mean they're expecting you to send more than one...it's just a general suggestion.