If you have multiple proposed shipments routed to the same FC, there are a few possible reasons why:
Due to recent changes with Amazon's API, shipments are limited to 200 unique MSKUs per shipment. This means that Amazon will automatically split shipments up when they reach this limitation, and you will see two shipments routed to the same FC when this happens.
Note: This limitation only applies to the number of unique MSKUs allowed in a shipment. There is no limitation on how many units per unique MSKU can be added to a shipment.
When listing through a third-party service such as InventoryLab, Amazon splits the shipments for any items that have different label types. They will not allow you to combine NO_LABEL, AMAZON_LABEL, and SELLER_LABEL into the same shipment in Stratify, even if they are routed to the same warehouse.
If items are routed to an FC that accepts hazmat (dangerous goods) items, but only some of the items are categorized as hazmat, they will be split into separate shipments from the non-hazmat items. This is because Amazon does not allow hazmat and non-hazmat items to be shipped together.
There is currently no way to combine shipments or add proposed shipments to existing shipment plans at this time unless you are shipping LTL using the Send to Amazon workflow. For more information, please see- Send to Amazon: Merge shipments
FAQ
What if I don't like how Amazon split my shipments?
If you get to the Proposed Shipping FBA Shipment page in a Private batch and don't like the splits, you can click the "Decline Shipments and Go Back" button.
Note: The option will not be available once you create any of the proposed shipments or add any to existing shipments.
This allows you to remove or add additional items to the batch before trying again to see if that makes the splits more cost-effective.
Can I delete some of my shipments?
Due to Amazon policy, you are not allowed to delete any shipments in a shipping plan unless you delete them all. For more information, please see- Deleted, misrouted, and incomplete shipments
This means that, if you need to delete a shipment, you also have to delete any other shipments that were created from the same shipping plan to adhere to this policy.
Steps to help with deleting shipments can be found here- How to Delete Working Shipments in Seller Central
Will I ever be able to have more than 200 unique MSKUs in a shipment again?
No, not unless Amazon adds the functionality to their new Selling Partner API.
Amazon has stated these changes were made to prevent adding additional items to shipments without planning. Unfortunately, our efforts in requesting this to be changed with Amazon have not been successful.
If you have any questions or concerns regarding the limitations of the Selling Partner API, you may contact Amazon’s Seller Support SPAPI team.