Depending on what time of day they pick up the mail and when you send it, it will probably take 1-2 days.
Instead of sending it from yourself, how about sending him a card from a Hogwarts professor suggesting that they met his sister and discussed his Hogwarts application, and hope they will see him at Hogwarts in the future? You could sign it Professor Snape (you said he was a Slytherin fan) or some other character. Or a message from the Slytherin Quidditch team, offering him a chance to try out.
If it's not from you, it won't matter if you beat the card home - plus, then you can see the look on his face when he reads it.
Or since you know him well enough, you could probably think of something else he'd like to hear from Voldemort, Bellatrix, the Malfoys, or some other character. Maybe even Harry or one of the "nicer" characters.
ETA: Or maybe a note from Mr. Ollivander himself? Dear Master __________, Thank you for sending your sister to pick up your wand. When I allowed her to wave it, she immediately came down with a case of Bat Bogey, so I trust that it will be perfect for you. Sincerely, Mr. Ollivander
If you could get someone else to write the message for you, he won't recognize the handwriting.