ADD PAYMENT email to client does not work anymore.

When I create an invoice it is created as a DRAFT until I SEND EMAIL or MARK AS SENT. When I click the MARK AS SENT button, the system does NOT email an invoice to the client. When I click the SEND EMAIL button it sends an email to the client with the invoice attached and the appropriate email as set up in settings.

ABOVE WORKS GREAT.
Now I receive a payment and the options are MARK PAID and ADD PAYMENT. I assume the MARK PAID does NOT email the client informing him/her that the payment was received. BUT I would like to inform the client hat his invoice was paid when I click ADD PAYMENT. Is this how it suppose to work? Neither MARK PAID nor ADD PAYMENT sends out an email to the client. As far as email settings in mail, it does work as when an invoice is created, the client receives an email.

Can anyone give me a pointer in the correct direction on how I can fix this please. Thanks in advance.