The job notice offered a "Flat Bonus" for picking up the shift. What does this mean?

The bonus is a fixed amount - not an hourly bonus.

Sometimes when there is a last minute job opening for a massage shift we offer a bonus to the therapist who accepts the job. This is posted as a "Flat" bonus which means that it is a set amount for the entire shift - not a bonus per hour worked.

If you accept the shift, you will receive your regular hourly wage plus the bonus.