The tax calculation is performed exclusively by the Avalara engine.

The add-on does not perform any kind of tax calculation, remember the add-on is a connector to integrate the Avalara calculation features into SAP.

If you calculate tax on a document, and receive a different tax amount than the expected, you can:

1. If the document is an invoice, check your Avalara admin console, and examine the calculated tax on the document, and confirm the calculated taxes.
2. If you are calculating on another document that is not posted to Avalara, like Sales Orders, you can use the Tax Calculator functionality at the Avalara admin console, to test the calculation. 
3. If you are use CertCapture with exemption certificates, and the exemptions are not being applied, contact your Avalara account manager to check your CertCapture account is linked to your Avatax account.

If you still think the taxes are not accurate, contact Avalara support or your Avalara account manager detailing the case.

If there is any problem regarding the information being sent from SAP to Avalara, or vice versa, then those are the kind of problems Argentis can jump in and fix.