He closed that envelope (which already had a name written on the outside), sealed it with more green wax, and pressed a final Sickle into that seal.
Then Harry put that envelope into the very last envelope on which was written in large letters the name "Merry Tavington".
And Harry peeked around the bend to where the scowling portrait that served as the door to the Slytherin dorms waited; and as he did not wish the portrait to recall not-seeing anyone invisible, Harry used the Hover Charm to float the envelope to the scowling man, and tap it against him.
The scowling man looked down at the envelope, peering at it through a monocle, and sighed, and turned around to face toward the inside of the Slytherin dorms, and called, "Message for Merry Tavington!"
The envelope was then allowed to fall to the floor.
A few moments later the portrait door opened, and Merry snatched up the envelope from the floor.
She would open it up and find a Sickle and an envelope addressed to a fourth-year student named Margaret Bulstrode.
(Slytherins did this sort of thing all the time, and a Sickle definitely constituted a rush order.)
Margaret would open her envelope, and find five Sickles along with an envelope to be dropped off in an unused classroom...
...after she used her Time-Turner to go back five hours...
...whereupon she would find another five Sickles waiting for her, if she got there quickly.
And an invisible Harry Potter would be waiting in that classroom from three PM to three-thirty, just in case someone tried the obvious test.
Well, it had been obvious to Professor Quirrell, anyway.
It had also been obvious to Professor Quirrell that (a) Margaret Bulstrode had a Time-Turner and (b) she wasn't very strict about how she used it, e.g. telling her younger sister really good pieces of gossip "before" anyone else had heard.
Some of the tension leaked off Harry as he strode away from the portrait door, still invisible. Somehow his mind had still managed to worry about the plan, even knowing that it had already succeeded. Now there remained only the confrontation with Dumbledore, and then he was done for the day... he'd go to the Headmaster's gargoyles at 9PM, since doing it at 8PM would seem more suspicious. This way he could claim that he'd just misunderstood what Professor McGonagall had meant by "afterward"...
The obscure pain clutched at Harry's heart again as he thought of Professor McGonagall.
So Harry retreated a little further into his dark side, which had worn the calm expression and kept the fatigue off his face, and kept walking.
There would come a reckoning, but sometimes you had to borrow everything you could today, and let the payments come due tomorrow.
Even Harry's dark side was feeling the exhaustion by the time the spiraling staircase had delivered him to the great oaken door that was the final gate to Dumbledore's office; but since Harry was now legally four hours past his natural bedtime, it was safe to let some of the fatigue show, the physical if not the emotional.
The oaken door swung open -
Harry's eyes had already been focused in the direction of the great desk, the throne behind it; so it took a moment to register that the throne was empty, the desk barren but for a single leatherbound volume; and then Harry shifted his gaze to see the wizard standing among his fiddly things, the mysterious unknown devices in their scores. Fawkes and the Sorting Hat occupied their respective perches, a bright cheerful blaze crackled in a nook that Harry had not before realized was a fireplace, and there were the two umbrellas and three red slippers for left feet. All things in their place and in their customary appearance except the old wizard himself, standing tall and dressed in robes of the most formal black. It came as a shock to the eyes, those robes on that person, it was as if Harry had seen his father wearing a business suit.
Very ancient was the appearance of Albus Dumbledore, and sorrowful.
"Hello, Harry," said the old wizard.
From within an alternate self maintained like an Occlumency construct, an innocent-Harry who had absolutely no idea what was happening inclined his head coldly, and said, "Headmaster. I expect you've heard back from Deputy Headmistress McGonagall by now, so if it's fine by you, I would really like to know what is going on."
"Yes," said the old wizard, "it is time, Harry Potter." The back straightened, only slightly for the wizard had already been standing straight; but somehow even that small change made the wizard seem a foot taller, and stronger if not younger, formidable though not dangerous, his potency gathered about him like a cowl. In a clear voice, then, he spoke: "This day your war against Voldemort has begun."
"What?" said the outer Harry who knew nothing, while something watching from inside thought much the same only with a lot more profanity attached.
"Bellatrix Black has been taken from Azkaban, she has escaped from a prison inescapable," the old wizard said. "It is a feat that bears Voldemort's signature if ever I have seen it; and she, his most faithful servant, is one of three requisites he must obtain to rise again in a new body. After ten years the enemy you once defeated has returned, as was foretold."
Neither part of Harry could think of anything to say to that, at least not for the few seconds before the old wizard continued.
"It need change little for you, for now," said the old wizard. "I have begun reconstituting the Order of the Phoenix that will serve you, I have alerted the few souls who can and should understand: Amelia Bones, Alastor Moody, Bartemius Crouch, certain others. Of the prophecy - yes, there is a prophecy - I have not told them, but they know that Voldemort is returned, and they know that you are to play some vital role. They and I shall fight your war in its lesser beginnings, while you grow stronger, and perhaps wiser, here at Hogwarts." The old wizard's hand came up, as though beseeching. "So to you, for now, there is but one change, and I implore you to understand its necessity. Do you recognize the book on my desk, Harry?"
The inner part of Harry was screaming and banging its head against imaginary walls, while the outer Harry turned and stared at what proved to be -
There was a rather long pause.
Then Harry said, "It is a copy of The Lord of the Rings by J. R. R. Tolkien."
"You recognized a quote from that book," said Dumbledore, an intent look in his eyes, "so I assume you remember it well. If I am mistaken, let me be corrected."
Harry just stared at him.
"It is important to understand," said Dumbledore, "that this book is not a realistic depiction of a wizarding war. John Tolkien never fought Voldemort. Your war will not be like the books you have read. Real life is not like stories. Do you understand, Harry?"
Harry, rather slowly, nodded yes; and then shook his head no.
"In particular," said Dumbledore, "there is a certain very foolish thing that Gandalf does in the first book. He makes many mistakes, does Tolkien's wizard; but this one error is the most unforgivable. That mistake is this: When Gandalf first suspected, even for a moment, that Frodo held the One Ring, he should have moved Frodo to Rivendell at once. He might have been embarrassed, that old wizard, if his suspicions had proven false. He might have found it awkward to so command Frodo, and Frodo would have been greatly inconvenienced, needing to set aside many other plans and pastimes. But a little embarrassment, and awkwardness, and inconvenience, is as nothing compared to the loss of your whole war, when the nine Nazgul swoop down on the Shire while you are reading old scrolls in Minas Tirith, and take the Ring at once. And it is not Frodo alone who would have been hurt; all Middle-Earth would have fallen into slavery. If it had not been only a story, Harry, they would have lost their war. Do you understand what I am saying?"