Are you on GoCentral? If so there is no way to change the Abandoned Cart email message at this time. 😞
That's not to say that won't be an option at some point in the future. This is a product, that based on my experience playing with my test account, is constantly evolving.
I have the same issue. I can't believe they would build this and not allow the customer to control the message that is going out to the customer.