ShipStation can automatically populate customs information for your products based on the product description and value from the marketplace. To be sure that this setting is enabled on your account.
Head to your Account Settings (the wrench icon in the upper right).
Next, select Shipping and then International Settings from the sidebar on the left.
Under the Default Customs Information section, be sure that the Customs Declarations drop down menu is set to Create declarations from order items.
Now the product description and value from the marketplace will display on the customs form.
If you would like to override the product description and value from the marketplace, you can do this by setting up product defaults for your items.
In the example below the original product name was 40 cm Necklace Chain under Product Information, but you can simplify the information by entering Necklace as the Description within the Customs Declarations tab. Also, you'll need to select the country where the product was manufactured as the Origin Country.
Now ShipStation will override the marketplace description with your updated information. You can update this information in bulk as well, by importing your customs declarations to the product defaults.
If all of your products can fall under the same description, simply choose Use Pre-Defined Values in the International Settings and enter in the customs information there. This will override all product defaults and item information and make your customs declarations the same for all orders.