If it's lowered, they'll refund you the difference.
E.g.: If you bought a MacBook, and later that week the price on Bestbuy’s website dropped by $200, they will refund you around $215 with taxes included. Very good customer service, only caveat is that they make you wait a bit to verify the difference.
They also match prices with Amazon, so if you find the same item with a lower price on Amazon, they will drop it for you