We currently support carriers and their associated rates that are available through the EasyPost, Shippo and ShipStation platforms.
EasyPost currently gives you 120.000 shipments per year for free
Supported Carriers: https://www.easypost.com/carriers
Supported Service-Levels (important): https://www.easypost.com/service-levels-and-parcels
Supported Carriers: https://www.shipstation.com/partners/integration/carriers/
For the US sign-up for a ShipStation account here https://www.shipstation.com
For the UK sign-up for a ShipStation account here https://www.shipstation.co.uk
ShipStation does currently not support the majority of dedicated return-rates through their API, this includes creating return labels with but not limited to:
Australia Post eParcel Returns
RoyalMail Tracked Returns
Note: in most cases you can use the regular outbound rate of the carrier to create a return label and we reverse the addresses automatically.
Missing return labels is an open issue with ShipStation in their forums: http://feedback.shipstation.com/forums/330429-product-feedback-fresh-ideas/suggestions/12873420-create-return-label-via-api
Do you need your own carrier-contract? See Do I need my own carrier contract?
Can you use your own carrier-contract? See Can I use my own carrier contract with pre-negotiated rates?
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 and others) already baked in. EasyPost is simple to use as well, currently comes with 50k free shipments per year but has only a USPS rate built in.
We build new carrier integrations on request as custom-projects for Shopify Plus merchants. If this is an option for you and you have a budget to accomodate development cost, reach out to us.
You still have the option to manually upload labels for each return in our dashboard. See also Using manual prepaid return-labels.