Items may be splitting into new shipments for a variety of reasons.  The most common is because Amazon determines which items go to which Fulfillment Centers (FCs) items will go based on the current inventory at the various FCs of the particular item. We have no control over FC assignment. Amazon does this in an effort to spread out inventory in order to get purchases to customers as quickly as possible.  An alternative for this is explained below.

A second cause for an unexpected split shipment is that the Ship_From address was typed in incorrectly or that it was different than that of the previous item.  In ScanPower LIST each item requires a Ship_From location to be selected.  This feature provides users with greater flexibility when accounts have multiple users working from different addresses.  However, items from different Ship_From locations cannot go into the same shipment resulting in a split shipment.

The third most common reason for split shipments is different prep_types.  You will see on your main LIST screen that each shipment has a prep_type (SELLER_LABEL, AMAZON_LABEL or NO_LABEL).  Items of varying prep_types cannot be in the same shipment and for this reason shipments will split and can even be assigned to the same FC.

The alternative for split shipments not based on prep_type is to turn on inventory placement in your Amazon Seller Central account. For a small fee, Amazon will let you ship the entire quantity of an item to one warehouse. The risk that you run is that Amazon will reassign your inventory and during this time your inventory will not be available for sale. This does not always happen, but it could. 

However, please understand that this will not prevent split shipments entirely.  Inventory Placement is item based which means it will not split the quantity of an item but it may split a different item.   For example, you have item A with qty 5.  With IP enabled, the entire qty of item A will be assigned to the same warehouse; however, when you add item B, it is possible it could be split to a different warehouse.