When you attempt to get a rate for a fulfillment through FBA, if you haven't added a Fulfillment or Seller SKU to the product details for the items in the order, or the SKU doesn't match the SKU in your FBA account, you will get the following error message:
"The request must contain the parameter Item.SellerSKU."
To fix this, you need to add the correct Fulfillment SKU to the product's details.
First, click on the product link in the Order Detail page.
In the Product Detail window that appears, enter the appropriate SKU into the Fulfillment SKU field (for FBA products, this should correspond to the MSKU value for your product in Seller Central).
Check the option to Apply changes to open orders, then click Save Changes.
ProTip: You can import all of your product records into ShipStation using a specially formatted CSV file, and use the FillSKU field for the fulfillment SKU. This will ensure all your products are updated using the correct Fulfillment SKU in ShipStation.