This paper develops two separate models for simultaneous determination of a product's manufacturing batch size and the order quantities of the input items that are used in the production process, in response to periodic, fixed purchase orders from a single customer. The effects of work-in-process inventories, resulting from a finite rate of production, on the producer's cost function is explicitly taken into consideration in the model development process. The first of these models assumes that all input items are procured lot-for-lot, based on the production schedule of the output item. This assumption is relaxed in the second model, where multiple procurement lots for any input during a production cycle are allowed. While an optimization procedure is outlined for solving the first model, for the more generalized case, an iterative, heuristic solution algorithm is proposed due to the associated computational burden, particularly for a large number of input items. These concepts are illustrated through a numerical example.