Email is fine, but avoid texting. In typical circumstances, a letter of condolence should be written by pen on paper and sent in the mail. But there is little that is typical about current circumstances.
First, you may not have stamps available, though you can download postage from websites such as stamps.com, and print it from your personal computer.
Or you may want to be sensitive to fears, justified or not, that paper sent through the U.S. Postal Service might be contaminated. (“After collecting mail from a post office or home mailbox, wash your hands with soap and water for at least 20 seconds or use a hand sanitizer with at least 60 percent alcohol,” the C.D.C. advises.)
In this case, the act of sharing your sentiments is more important than the medium you choose. Still, if you opt for digital communication, email is preferable to text or direct messages. Many mourners save condolence letters and turn back to them at anniversaries and other times of reflection. Emailed letters are easily printed, filed and saved.
Article source: https://www.nytimes.com/2020/04/09/style/best-way-to-write-a-condolence-note-coronavirus.html