When generating First Class Mail International shipments USPS will either generate a customs number or a non-trackable number. If you want these submitted to your Marketplace to show proof that your order was shipped even though the numbers are actually not valid tracking numbers, you will need to make sure you have this option checked in your ShipStation settings.
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.
At the bottom of this page there is a section for "Other Settings". Find and then check the box here that says Send non-trackable Tracking Numbers to Marketplaces.