Using an external facing web portal to leverage the AvaTax service.
AvaTax SOAP API
AvaTax REST API
No whitelisting should be needed to send information to AvaTax. Depending on your network set up, you may need to whitelist the AvaTax URL that corresponds with the environment that you are using:
To determine if any whitelisting is needed for inbound traffic, you will want to consult with your IT department.