I Know you need some kind of insurance to do this job in case you get stopped. But I'm not really sire how it works?
Nope. You tell them you want to use the vehicle for a delivery job.
They will most likely tell you they'll cancel you if you do.
You need a commercial insurance policy for this. If you are delivering items in your personal vehicle, stop right now…
If you do this once, no one cares. If you do this daily, you're doing business with your car, and if you're insured as a leisure driver you won't be covered when things go wrong.
You just put the policy for business use and leave it that way until you quit the job. You do not change it every day, you just leave the policy as business use until you leave the job.