Well the main reasons for not going horizontal are scale and volatility - which ammount to fixed costs essentially. If they can make a profit off of it then tha

Well the main reasons for not going horizontal are scale and volatility - which ammount to fixed costs essentially. If they can make a profit off of it then that implies cutting out the middlemen can potentially reduce expenses. That said that they are /still/ smaller than Wallmart yet have scale sufficient to be competitive to mass shippers is amazing even considering the internet related physical letter shrinkage.