Split shipments to same warehouses
Occasionally Amazon will split your shipment with multiple products going to the same warehouse but on different shipping plans. This is not ideal as it increases your spend on postage and generally seems off or unfair from the sellers perspective. It can look like the picture below when this happens.
This can happen for various reasons, but as the picture indicates, it's typically a labeling designation issue.
In certain circumstances, like if the product is in NEW condition, Amazon will want to allow that product to be commingled and label it themselves or just use the UPC barcode (manufacturer label) rather than the seller's label.
In an effort to prevent Amazon from doing this you can turn off the commingling permission in Seller Central directly.
Once inside, navigate to Settings > Fulfillment By Amazon > FBA Product Barcode Preference
Hit edit and make sure that your setting is set to Amazon barcode.
This should reduce the number of times this happens to you, but not all the time.