What type of relationship exists between suppliers and department stores?

At the most fundamental level, department stores are a distribution channel that facilitates the transaction of goods from supplier to the end customer. By providing a pleasant atmosphere, sufficient parking, convenient locations, security, etc. the customers' experience shopping is made a pleasant and enjoyable one.

Suppliers must pay for shelf space in department stores, with the "prime real estate" being the most expensive. (This is why items placed on the rack at the checkout are relatively expensive on a cost to value ratio, the premium will go to covering the shelf space cost)