PS at the end of a letter means ''after writing''. It is a Latin phrase that is meant to indicate that the text marked with 'P.S.' initials was added after the other content had already been written.
1 Additional Answer
P.S stands for 'post script' from the latin word 'Post scriptum'. It means that there is something more you want to say that has not been mentioned in the letter. It could mean an afterthought or extra note.