Salary is literally extrinsic motivation. The problem isn't bonuses, the problem is when bonuses become so regular and frequent that they turn into an informal part of the salary and not paying them becomes equivalent to withholding part of the salary.
If you pay a one-off bonus for outstanding work, make sure you tie it to something specific the employee can understand. If it's a reward for a massive deal decide in advance if you want to pay out similar bonuses for similar deals in the future and what the rules are.
An employer-employee relationship is not a friendship no matter how much you like each other. It's a dependent relationship and it needs rules to protect both sides. A five figure bonus is a big deal for many employees so treat them with the dignity they deserve by making sure they can understand the conditions. If you think there aren't any that just means you haven't spelled them out or don't want to be held accountable.
