This study presents an inventory model that leverages substitution flexibility, a strategy often beneficial in multi-product inventory systems.Specifically, we focus on a scenario featuring two substitute products (i.e.smartphones and tablets) and negligible lead times.We introduce stochastic parameters to represent customer demands for both products.Also, we incorporate queuing theory, a powerful analytical tool for optimizing inventory systems.Our goal is to minimize the combined costs associated with ordering, holding inventory, and the simultaneous ordering process.To achieve this optimisation, we employ the ABC-GOA (Artificial Bee Colony-Gravitational Optimization Algorithm) technique, a state-of-the-art optimisation approach.This methodology is applied to streamline the supply chain, reducing ordering costs, holding costs, and enhancing efficiency in the simultaneous ordering process.The result is verified by real time examples.