There are several legal requirements that you must follow in order to send mass text messages to contacts and this is one of them. This is for compliance purposes.
Do I need to add it to my SMS messages?
No, we will automatically append this to your SMS messages. You must provide a way for the recipient to "opt-out" of your messages and we do this for you so you don't have to.
Can I edit the "Reply STOP to end" message?
Unfortunately, we have this as a default message within your SMS messages. Currently, you cannot edit this on your account.