They set the bottom price, and if they are earning 2%-4% profit margins, then their costs increases will have to end up in either prices increases or them no longer selling the product.
The funny thing is that it is Amazon coming along and greatly increasing demand for labor (in conjunction with reduced supply of that type of labor) that caused all the other warehouse jobs and retailers to increase pay.
And I hope the labor continues making gains in their quality of life at work and pay, but I do not see this happening without an increase in prices for products (like we have already seen).