Except for the fact that a letter came with Harry when he was dropped at the Dursleys. Right?
Or am I misremembering?
I was surprised that there was nothing else in the book re. Petunia's childhood correspondence with Dumbledore. I also was surprised that apparently some contact remained between the sisters, as Lily refers to baby Harry breaking a vase that Petunia had sent to Lily for Christmas. I had the impression that there was no contact. I also wonder what happened to the Evans parents? Lily was 21 when she died, so they probably were not that old, yet it seems both must have died.
The letter Petunia sent to Dumbledore would have been long before Harry was ever born.
No, you're right.
But a lot of folks have wondered why a howler from Dumbledore, saying "remember my last," would affect Petunia as it did. "My last" seems to imply more than one letter was exchanged between them. Book 7 fleshes it out a bit more: she had actually corresponded with Dumbledore before, begging to be "taught" magic. It helps to explain why Petunia was affected more than perhaps she ought to have been by a seemingly cryptic message.