InventoryLab does not control Amazon shipment splits. The presented shipments come directly from Amazon and are based on your Inventory Placement settings. If you'd like your items to go to one location, you will need to select Minimal Shipment Splits within your Seller Central settings. With this option, Amazon will typically distribute your inventory to one location, after which it will redistribute it to other warehouses for an extra per-item fee. More on this, including exceptions, can be found here.