For heterogeneous samples such as polymer pellets, it is critical to obtain a measurement that is representative of the bulk sample and not just a small fraction of the material. This is often a significant challenge when using traditional near-infrared spectroscopy sampling methods. Accessories such as the Sample Cup Spinner allow a greater amount of material to be analyzed in an automated device. In this study, two diffuse reflectance-sampling methods were compared to determine the most efficient and accurate method for sampling polystyrene pellets. A single calibration model was developed to determine the concentration of an ultraviolet (UV) stabilizer additive in polystyrene pellets. Using the two sampling methods, the concentrations of four unknown samples were determined using the single model. The results demonstrate that the Sample Cup Spinner accessory provides the optimum performance with the shortest analysis time.