Buffy never knew about the letter.
Never knew that there was a girl out in Italy who had it stuck in the bottom of her underwear drawer, hidden from an immortal sex god.
And because she never knew, she never spoke of it.
And because she never spoke of it, Spike never spoke of it either.
He never asked her why she hadn't come to help them in Los Angeles, or why she never called, or why she had never asked for his help. He never asked her if she even knew how much he loved her still.
And because Spike never asked any of these things, Buffy never knew. She never knew that he had tried to contact her, never knew he'd been prepared to die in a rainy alley in Los Angeles, thinking she never loved him, never knew that he had needed her help. Never knew how much he still cared.
And because she never knew, she never called out for him when he left her on a rooftop and flew away.
She just never knew.