If your product SKU doesn’t match the Amazon SKU for FBA or Shipwire, you will need to update the product details in ShipStation with the Fulfillment SKU for your products. You can reach this by clicking the Product Details in an order’s detail page, or clicking on a product from the Products tab.
Click on the Shipping and Customs section, and then fill in your Fulfillment SKU from Amazon or Shipwire for that specific product. and click Save.
Once you've entered a value, Save Changes.
You can also import all of your products with the column “FillSku” filled out to be sure all of your products have valid SKUs for your fulfillment provider.
Note: This information only applies to those using Fulfillment by Amazon or Shipwire. If you're using both, you'll need to make sure both of them are using the same Fulfillment SKU.
For Fulfillment By Amazon, the value that needs to be used is the MSKU value.