Increasing awareness of sustainability and regulatory pressure drive the use of product-specific returnable transport items (RTI) instead of disposable packaging. However, little attention has been paid to decision support for the joint optimization of economic production and order lot sizes, with the simultaneous calculation of RTI fleet sizes, considering different modes of ownership in a multi-tier supply chain. This research models a three-tier supply chain consisting of a component supplier, a manufacturer, and a customer. The aim is to determine the joint economic production and delivery lot sizes, while simultaneously determining the required RTI fleet sizes for both the supplier’s and the manufacturer’s products, considering different cost components for RTI (rent, investment, or a combination thereof). In addition, capacity constraints must be respected. To achieve these objectives, a MINLP is developed and solved. Numerical examples are provided to demonstrate the model’s behaviour.