I'd be glad to help check why your sales tax charged in eVet is different that what is calculated in QuickBooks Online (QBO).
Sales tax is a retail point-of-purchase tax imposed by state and local governments that is paid by the purchaser for goods and services. In QBO, sales taxes are calculated based from the United States Portal Service (USPS).
To check how do eVet calculates sales taxes, you can contact with the eVet Support.
In addition, you may get in touch with our Intuit Developer to check this further for you securely.