A real performance guarantee is only useful if it tells you exactly what the agency will do, how you will judge it, and what happens when the target is missed.
Plain English or no deal
If a Klaviyo email marketing agency cannot explain the guarantee in plain English, walk.
You should not need a translator to understand the price, the scope, or why the work is supposed to pay off.
The baseline has to be defined before anything starts.
That means your current email revenue, repeat purchase behavior, list quality, deliverability, and the current state of your flows and campaigns.
Not when the agency wants to move the goalposts.
If they cannot tell you the starting line, the promise is smoke.
A good guarantee also spells out the scope.
How progress is measured.
And what happens if the target is not reached.
At Loyal Send, our guarantee is blunt, 20% email revenue increase in 90 days or we work for free (newsletter_ops).
That is the kind of language you want.
The guarantee should cover the whole retention system