A Shipping Plan is Amazon’s proposed plan for splitting your inventory between fulfillment centers. A shipping plan is automatically generated by using the send/replenish button on the Manage Inventory screen of Seller Central. You have the option of accepting the shipping plan or cancelling it. There is no penalty for cancelling a shipping plan.
To view a shipping plan in your Amazon Seller Central account, go to Inventory - Manage FBA Shipments - Shipping Plans.
A Shipment, on the other hand, is created when you accept a shipping plan. Sometimes a shipping plan will create multiple shipments. At this point, you are committed to following through with each of the shipments indicated. Although shipments can be cancelled, this should only be a last resort, since Amazon does penalize sellers who repeatedly cancel shipments.
You can view your shipments by going to your Amazon Seller Central account. Go to Inventory - Manage FBA Shipments.