Blizzard is pretty comfortable admitting that the late game is the strongest point in World of Warcraft, and the new Warlords of Draenor expansion will include an instant boost to level 90 so you can skip right to it. The studio seems to have figured out that people might pay for such convenience, and has announced plans to test that soon.
A post from the Battlenet blog reviews the pre-order bonuses, but near the end says that soon it will be testing a feature to let players purchase the character upgrade boosts directly. It says it bases this on feedback from players, who say they want some way to quickly bring their alts or characters on other servers up to snuff without buying multiple copies of Draenor.
If you can make do with just one insta-90, you'll actually be able to get it before the expansion is even released. The blog says that pre-purchasing either of the digital versions will get you the boost right then. Pre-purchasing the Digital Deluxe version will similarly net you the exclusive pet and mount upon purchase.
Blizzard says it's giving out the bonuses early thanks to feedback from players who would like the chance to get acquainted with their level 90 heroes before venturing into the Draenor conflicts. Draenor will raise the level cap to 100, so you'll still have some grinding to do.
A closed beta test is scheduled to begin sometime soon, and Blizzard dropped a helpful reminder to opt in and make sure your profile is up to date if you want to be eligible.