Pingdom has crafted a wonderful example, based on the Google SLA given to its cloud customers. Google's SLA: "'Downtime Period' means, for a domain, a period of ten consecutive minutes of Downtime. Intermittent Downtime for a period of less than ten minutes will not be counted towards any Downtime Periods." Pingdom, correctly pointing out that frequent short outages are much more common than long ones, did the math and reported that "Gmail could be unavailable for more than 21 hours in a day and Google could still tell you that, according to their SLA, the service has had 100 percent uptime." You really need to read that piece before you sign your next SLA.
A service level agreement (SLA) is a guarantee from a site that its uptime will be as promised, n'est pas? Actually, quite the opposite. It's more likely a complicated document designed to protect the site (not the retailer) in case of any problem.