The application of the Automated Storage and Retrieval System (AS/RS) in the warehouse has been improved significantly over the last few decades. In the recent years, a new type of AS/RS, Shuttle Based Storage and Retrieval System (SBS/RS) has taken many industries interest by storm due to its flexibility and throughput performances. This paper studies the mathematical optimization of item location assignment and task scheduling in SBS/RS mainly with tier-to-tier system which allows the shuttle to transport between tiers using a lift system. Since this system is mostly applied in giant e-commerce industries, considering order data set and multiple stock keeping unit (SKU) in the mathematical model formulation will give a significant benefit in the real-life scenario. This paper also proposed a modified genetic algorithm to solved both item location assignment and task scheduling with a good computational time result for large size instances within a reasonable amount of time.