Good one. This will depend on the integration you have set up, be it Shippo or EasyPost. Shippo and EasyPost create the required customs-docs/commercial invoices and pass them alongside the label. Depending on the specific carrier, rate, and country the customer is located in - these PDFs will vary in their content.
What Rich Returns submits to EasyPost and Shippo is roughly the following for international shipments:
Item country of origin
Item HS code
Item value in USD
Contents type: returned goods
Contents explanation (if carrier supports it we include the tracking number of the original outbound shipment, the carrier that was used and the original order number)
Customs Number / Exemption
For certain carriers (e.g. UPS and DHL Express) there is also a Paperless-trade option available where Customs-Information is only submitted electronically without printing the form.