Taxjar SalesTax

Open one of these files (or both, if you have both):

app/code/local/Taxjar/SalesTax/etc/config.xml
app/code/community/Taxjar/SalesTax/etc/config.xml

Find following lines:

<helpers>
    <taxjar>
        <class>Taxjar_SalesTax_Helper</class>
    </taxjar>
</helpers>

Replace them with:

<helpers>
    <taxjar>
        <class>Taxjar_SalesTax_Helper</class>
    </taxjar>
</helpers>
<sales>
    <quote>
        <totals>
            <tax>
                <class>Taxjar_SalesTax_Model_Sales_Total_Quote_Tax</class>
            </tax>
        </totals>
    </quote>
</sales>

Save the file and clear magento cache.