Agree with formerfed. Some lessons learned from doing a few of these:
Keep it simple.
Slant it towards customer satisfaction, not for completion of tasks that are required anyway. That means it will be subjective; so what? That ;s the point of building the partnering relationship. Some agencies have created huge bureaucratic exercises over the award of a simple fee which disincentives the use of it except in really large contracts, and forces everything towards paying rater than not paying. Actually they are very useful in smaller contracts. In the cases where I did these, the vendors who were into customer service made remarkable changes when they didn't get their first incentive payment, and the vendors who were just into extracting money never did make the changes and in some cases seemed happy being terminated for cause.
Weight the fee payments to incentivize continued excellence, e.g. 1/3 at the middle and 2/3 at the end of the work. Many vendors try to straight-line the fee so that they have collected most of it as early as possible, at which point the customer loses the leverage they were trying to buy