 
Apple does not authorise sales through Mail Order.  As a result mail order
companies have to obtain their machines by the grey market.

This market is supplied with machines from authorised resellers who have
more machines than they can sell.  They come into this state of affairs
by overordering either accidentally or deliberatly to get a better
wholsale price from Apple.  In either case they often obscure the serial
nunber to protect their identity.  As a result the warranty is void.

You may save on sales tax but you have to pay for shipping.

I should also point out that mail order companies cannot get lower prices
thatn the high volume authorised dealers unless they buy below dealer
cost.  As a result the only way they can sell cheaper is by cutting costs
and trimming margins.  Unfortunatly they dont seem to be doing too well. 
The lowest prices I have been quoted mailorder do not beat the lowest
prices available from authorised local dealers.

They are even further away from educational prices.