Beside the Golden Throne of the Emperor of man! If not there then Liverpool
Its certain the free boost will be before WoD so I would assume the paid service would be too as I am guessing Blizz and players would want people to get used to their new class before they dive into Draenor?
They've said previously that when WoD is available for digital pre-order, people who pre-order it that way will be able to claim their free 90 boost immediately. I presume that the paid boost will be available at that same time or shortly after.
They've said they're considering letting people who prepurchase WoD to use the included character boost instantly. The character boost you can buy from the shop has had little to no confirmation or information released.
My personal guess is that they'll allow the prepurchase boost whenever prepurchase goes up. Then they'll enable the shop boost when 6.0 is released. That would just make sense to me from a monetary perspective. Maximize presales, then open up shop boosts once class info is finalized in 6.0.
People will certainly buy multiple WoDs to get multiple boosts before the shop boost, but that'll cost people even more money and Blizzard is probably fine with impatient people since they fork out more money.
The reason it was in 5.4.7 (and momentarily showed up) is so that another patch wouldn't be necessary to implement. My guess is that it'll be available the same day as WoD pre-orders are available (since the free 90 boost starts then).