And a month later they have competition. The barriers to entry in online retail are very low. Anyone can sell online and can scale their operation from small to large with little difficulty. A little guy can't do same-day delivery but almost any traditional retailer can.
No one is going to be able to match their economies of scale. That is entirely a function of how much product they move, and once a threshold is achieved, it will then become impossible to offer prices lower than the behemoth who currently has most of the market.
They can reduce their profit margin lower than your cost of doing business.