As a ginger myself this is outrageous.
Whilst I was never really bullied as such at school, mainly because I was a typical feisty ginger, there were frequently lewd comments made about matching collar and cuffs and all that, which I found particularly excruciating throughout my teens, and comments about white freckly skin. I find that people only begin to love red hair in this country as adults, and that is purely because it is massively socially unacceptable as a child and teen to like it, and so the cycle carries on.
I say this country, meaning England, it is absolutely fine to be a redhead kid in Scotland, Ireland and the USA, Sorry Wales - not sure what the stance is there.
For a total stranger, and an adult at that wearing a Royal Mail uniform, to make a gingeriest comment such as that about a child I would be furious. In fact I am furious now.
Complain.
PS I detest the word Ginger, I wouldn't say a blonde had Custard hair, or a brunette Mud hair so why is Ginger OK?