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:
Sender (Customer)
Recipient (Company)
Item description
Item country of origin
Item HS code
Item value in USD
Item weight
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)
Signer
Customs Number / Exemption
RMA number
Order number
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.