I would be concerned about any sort of individual bonus directly linked to the service just rendered. That tends to work against cooperation and creativity and problem solving. Only when you are unencumbered by incentives can you afford to experiment and learn from your mistakes and get better at what you do. (See e.g. Pink's Drive for more details on motivation.)

It's especially bad if it's based on some variant of having scores "above average" or whatever, because that ends up being a lottery. At that point it's better to just establish a lottery among the employees. If you really have to do individual bonuses, I think it's better to take a leaf from Deming and only grant it for truly (statistically verified) outperformance.

Someone else suggested collective bonuses (e.g. profit sharing). That aligns incentives nicely: it is slightly lagged, meaning you can afford to experiment and learn. It's also not individual, so it fosters collaboration. It's still something of a lottery, of course, but one decided by the customers, not by a manager.