The rapid growth of the e-commerce has led to a dynamic increase of shipped parcels in recent years. Operators of transshipment terminals face the challenge to quickly sort and transfer parcels in order to successfully compete on the market and to meet their customers' expectations. A key factor to operate on high efficiency level is to provide optimal assignment decisions in the allocation of existing resources (e.g. unloading dock assignment, sorting destination assignment). We present a solution approach that closely links mathematical optimization and discrete-event simulation in an iterative way. In particular, this paper investigates the impact of different objective functions on the terminal system performance. Computational results are presented for two different transshipment terminals.