Order fulfilment systems are forced to manage a volatile customer demand while meeting customer-required short order deadlines. To handle these challenges, we introduce the Strategy of Levelled Order Release (LOR) for workload balancing over time. The contributions of this work are (1) the workload balancing concept LOR, (2) a discrete-time Markov chain for performance analysis, and (3) an algorithm for capacity planning under performance constraints in order fulfilment systems with LOR.