Picking operations inside a warehouse are the major share of the total costs of retailing operations. Optimization and harmonization of operations is therefore crucial in the economy of a successful business. In this paper we consider the case of a leading German grocery retailer company and we adapt to their case a Mixed Integer Linear Program solving the order batching, assignment and pickers routing problems. Improvements to the model are also discussed, Computational experiments are finally presented, validating the different models and ranking them in terms of their applicability to real scenarios.