At high volume, prep center mistakes become expensive fast. A mislabeled shipment at 5,000 units costs far more than the same error at 100. The evaluation criteria changes once you hit serious scale.
Look for centers with dedicated workflows per platform. Amazon FBA and Walmart WFS have different requirements, and a center that handles both in one undifferentiated queue is a risk at volume.
Ask about container capacity. If you are importing full containers, the 3PL needs to receive, sort, and stage large inbound without bottlenecking your prep timeline.
Same-day dispatch capability matters. At volume, inventory sitting an extra day in the warehouse has a direct cost. Ask specifically whether same-day qualifying outbound is available and under what conditions.