How can dollarama's sell items so cheap when you can get the same item for more cost at like walmart or canadian tire or etc?

Each store can sell items at the prices they want. Let's say both Dollarama and Walmart each get Poland Spring Water Bottles. Poland Spring will sell it to both stores for a cheap price like 20 cents a bottle. Now, Walmart and Dollarama want to make money, so they have to raise the price to make a profit, but Dollarama might sell it for a dollar while Walmart sells it for two dollars.