There is a difference between a service guarantee and a service level agreement (SLA). An SLA is about metrics that measure how well the service is doing. When those metrics are not met, an SLA means credits must be offered.
I have mentioned this numerous times but an SLA is No Guarantee. An SLA from the Big Boys is a pain in the butt to collect on. You have to jump through hoops to see credits. And here's the funny part: on a $600 per month service, the SLA is $20 per day!!!
I would offer a pro-active SLA. We monitor everything. Want the reports? You can get them through the customer portal or that dashboard we built or we can email them to you daily (for $1 - is that okay?) When something goes wrong, we tell you. We tweet it (twitter.com/awesomeclec), text, IM, email, and call you if you want. And we will proactively give you your billing credits. That is our promise to you.
A Service Guarantee is when you absolutely deliver on your promise or the customer can walk. For example, we will install within 21 business days. Or Broadcore's 60 day free trial. You pay on day 61 if you are happy.
There are many examples of SLA's. In fact, the SLA for the Big Boys is usually included in the MSA (Master service agreement or contract).
It's a way to be different! And what a story to tell.