SF Expres is an International Carrier serving US, Asia, and European markets. For detailed information about their service area coverage, you can use their service area lookup tool here. If you have an interest in using their services you can sign up on their website at https://www.sf-international.com/us/en/dynamic_function/guide/register/index.html.
Once you have signed up for an SF Express account they will provide you with Test and Production API Credentials. You can use those credentials to add a carrier account with EasyPost. Once you do that you are ready to create test and production SF Expres shipments via the EasyPost API.