The Z-Quotes/Z-Force integration is not calculating tax correctly. The tax is too high.
The AvaTax for Z-Quotes integration lost the ability to pass the tax code after a specific release of the Z-Quotes package. This means the integration will need to rely on item mapping (based on the Z-Product SKU field) for taxability.
Note: if the taxability needs to be different for different charges within the same SKU, this integration may no longer function for your needs. You may need to contact Zuora to get their latest integration with Salesforce, which will include an AvaTax component.