EDIT: I've represented a whole lot of customers of web services over the years (IAAL). The big lesson in this area for me is: (1) customers rarely invoke SLA credits, preferring instead to "work it out" at the relationship level, and (2) most provider off-the-shelf SLAs are so full of holes and tricky thresholds that they are effectively useless. On this last point, beware the 100% or five nines or other unreasonably high uptime commitment. When you get into the details of the SLA (the demarcs, the qualifications for obtaining credits, the remedies for failure), you will almost always find that there is no realistic remedy at all.

N.B. Meant to edit my comment above, not self-respond.