We currently support carriers and their associated rates that are available through the Shippo and ShipStation platform.
Shippo carrier list: https://goshippo.com/carriers/
ShipStation carrier list: https://www.shipstation.com/partners/integration/carriers/
If you want to get started quickly we recommend setting up a free Shippo account. It is dead simple to use, pay as you go and has some commercial rates (e.g. USPS) already baked in.
We do build new carrier integrations on request as custom-projects for merchants from time to time, especially for Shopify Plus stores. If this is an option for you and you have a budget to accomodate development cost, feel free to reach out.
You still have the option to manually upload labels for each return in our dashboard. See also Using manual prepaid return-labels.