He knew it was a bit weird sending a letter of all things through the veil, but it made him feel a little bit better every time he did.
Its not that he expected it to be read or anything, he just could not put what he was feeling into verbal speech. He did feel that Sirius needed to know though... He needed to know that he didn't die in vain.
The first letter was sent the day after Voldemort fell. The night of the defeat of Voldemort, there was a big celebration. The newspapers were all over the place. They had won. They never did mention all they had lost...
Harry was sitting in front of the veil, with a letter in his hands. It was a normal sized letter in a normal sized envelope, cream in colour. The letter was not a long one, per se. Just enough syllables to explain that the war was over. Harry almost expected it not to go through at all, but it did. It slid through like a hot knife through butter.
The following morning, the same letter was sitting on his dresser, unopened, with the words 'return to sender' stamped in red on the front.