When my son was born, it was clear he had taken after my dad with a shock of ginger hair. Now, my dad had been called Ginge and Rusty all of his life, and he smiled and got on with it, like water off a ducks back, and I wanted my boy to be the same. I named him a lovely normal name, however, all his 14 years, I have called him Ginge, Ginger, Red, Rusty, Copper-Nob, anything with the colour red in it, and he has never ever been bullied. My theory, rightly or wrongly (with us I was proved right!) was that if I always called him ginger and all the other names I could think of with his hair, then by the time he got to school, if anyone else started calling him these names too, then surely, he would think "So what, thats what my Mum calls me!"
And it worked! Once I think he came home and said that some lad had called him Ginger, but he took it as a compliment lol, and they became best mates!
My Ginge is now 14, he is so used to people commenting on his hair, either being daft like your postman telling him to buy hair-dye, or older folk telling him what a gorgeous colour it is. He couldn't care less. its his hair, and he knows that Gingers are becoming more rare, (true apparently!) and he just gets on with it.
I think if you make a big deal out of this, you're setting yourself up for a shock. There are a lot of folk as dappy as your postman who will comment, its the way of life for red-heads, and you can either arm your kid and make her confident and proud of this so she can let it go, or you can take every remark personally and your kid will follow suit, and let it rule their life.
Be thankful for the rare red-head, and whenever anyone says anything derogatory to you about it, you just smile at them, because our red-heads are all gorgeous! x