Just buy it twice. Buy it once now for the boost, and again later for the mouse mat, and if you ask nicely, Blizzard will be generous enough to refund the money you paid for the digital key. Or wait… what’s that? They don’t offer that refund? They’ll only refund the digital key and keep the money?
Well… if you’re not actively recruiting WoW players by buying them full-priced copies of the expansion that’s your own fault now isn’t it?