Whether you are writing a letter or sending an invitation, it is important to recognise the proper etiquette when addressing an envelope. This is especially important in delicate or tricky situations, such as how to address a widow.
Write your address in the upper left corner of the envelope.
Write the widow's name and address just to the right of the centre of the envelope. Unless the widow has remarried or changed her name, address her by the same title as before her husband's death, for example, as Mrs. John Baker.
Add postage (either a stamp or metered postage).