Just make sure you insurance carrier knows you use vehicle to deliver mail. If there is no exclusion on your policy for while you are delivering and you have an at fault accident is where the problems come. If there is no exclusion the PO will deny the TORT claim and say you have private insurance. In the end they will be responsible but it puts you in a very tough situation until it is worked out. Your insurance will not pay, the PO will not pay and this all leaves you holding the bag which at times takes legal action. So be sure your carrier is aware and has you covered properly.
I never told my insurance company that I used my own vehicle, however, that was a not bright move on my part
, luckily, I never had an accident ! And, never will 
!I retired after forty years and when I was working as a rural carrier, I told the insurance company how many miles I drove to work and back and that’s all I told them. I had one wreck on my route that wasn’t my fault and had no problem with my insurance. Now I’m not telling you to do that, but if you have an accident they don’t know if you were working or on your own time.
