Each request to the Single-Invoice.co REST API must be submitted over HTTPS. The majority of the requests will require a valid API key (the exceptions are methods like Register and Login which have their own separate requirements).
There are currently two types of API keys - API Account Management Keys and Application Keys.
Your API Account Management Key is created once your account has been approved. It provides access to the Account management functions; it should be used when you are performing actions like updating your account information or creating new API keys.
Coverage API Keys are used for the majority of core methods - for example, looking up a company, getting a quote, or requesting coverage.