The Cost+ API uses HTTP Basic Authentication over TLS 1.2+. Use your API key as the username with an empty password (include a trailing colon). The API endpoint is https://api.costplus.online/v1/ for both sandbox and production - the mode is determined by your website type in the Merchant Portal.
For the full guide including code examples, header construction, and HTTP status codes, visit our developer documentation: https://docs.costplus.io/docs/getting-started/authentication
Need help? Contact support@costplus.io.