ShipStation automatically submits your customs forms electronically for UPS (via Electronic Data Interchange, or EDI) and FedEx (via Electronic Trade Documents, or ETD). This service is enabled by default for FedEx users. Please note that UPS users must apply to UPS directly for this service, and once enabled on the UPS side, it will be enabled on the ShipStation side.
Electronic form submission then happens when you create the shipments for any orders that 1) require a customs form and 2) are being sent via either of these carriers. Unfortunately, we are unable to submit customs form information electronically for USPS, though your label information still does get transmitted.
When you print your UPS labels, there will be "EDI" printed on them signifying that you've electronically submitted your customs forms.
Likewise, we will print a "ETD" on your FedEx labels where we've submitted your customs forms electronically. (If this is not here, you will need to print the forms and send them on the package.)
If you see INV instead of ETD then that means the customs information will not be transmitted electronically.
If this happens, you will need to manually generate your customs forms. It is possible EDI is not enabled on your account. Please see article Where Can I Find My Customs Forms? to find where to locate and print your forms.
If you wish to have this transmitted electronically, reach out to UPS or FedEx to see what is needed to set this up.
For more information on processing international shipments with ShipStation, see this article.