Courier operators may have multiple drivers working for them, and may have new staff starting on a regular basis depending on what contracts are ongoing.
One of the benefits of courier fleet insurance is you can specify a minimum driver age so you do not need to notify your Insurer every time a new driver starts.
So for example you could have a policy that covers Any Driver 21+ meaning so long as the driver is over 21 years old and fits in with other common conditions (Minimum licence length, Driving Convictions) they are good to go!
(Always check your policy documents for the precise driving conditions set by your Insurer)