Yes, SKULabs supports Fedex One Rate with EasyPost! When creating shipments in SKULabs, you must provide the correct parcel dimensions and choose one of the predefined FedEx packages in the Packaging selection on the ship window before rating:

Most importantly, FedEx One Rate is not a separate FedEx service type. It is a flat-rate pricing tier when selecting eligible packages.

This means you won't see any separate FedEx services called "One Rate", but instead will continue to see your regular FedEx service types with FedEx One Rate prices when selecting from eligible packaging: