What is an expression that you can use at the end of an informal email, when you forgot something and want to add it to the email after your signature?
You can use either P.S. or N.B. For informal articles, P.S. is the apt choice.
P.S. being post scriptum, and N.B. (nota bene) if it is something important you want to draw the reader's attention to.
P.S. : In case of an email, the text can be altered, even if you forget to write something. And if you have already sent it, then you will have to send a mail again, including the matter you wanted to write.
By the way I forgot to add that... Forgot to mention that...
.... is perfect for informal emails.