I have Zero experience of the supermarket retail business so...

I have Zero experience of the supermarket retail business so maybe someone can explain further. Even though the retailer may offer a discount on the products to increase sales, how does this impact the profit for BAL? Do supermarkets not buy at a set price? Or is a percentage of the sale price sent back to BAL? Obviously more volume sold is beneficial but how does discounting affect the per unit profit?

Cheers