The AvaTax REST API exposes the most commonly used available for interacting with the AvaTax service, allowing calculation of tax, modification of documents, and validation of addresses.
Basic Auth is used for this API. Once you have signed up for an account you can use your details and make requests to endpoints with the authorisation in the request header
Authorization: Basic aHR0cHdhdGNoOmY=
This is your username and password base64 encoded