Hi,

To setup a tax type that is a fixed tax % on the overall invoice amount, you can follow these steps:
* Go to "Accounting" > "Configuration" > "Taxes"
* Click on the "Create" button to create a new tax.
* In the "Amount Type" field, select Percentage of Price.
* Set the "Amount" to 10.00 (since you want to charge 10% tax).
* In the Taxable Basis field, select Total. This will ensure that the tax is calculated on the total invoice amount, not on the individual line items.
* Save the tax.

Charging a fixed tax amount on the total invoice:
* Go to "Accounting" > "Configuration" > "Taxes".
* Click on the "Create" button to create a new tax.
* In the "Amount Type" field, choose "Fixed Amount" from the dropdown.
* Set the "Amount" to 10.00 (since you want to charge $10 as tax).
* In the Taxable Basis field, select Total. This will ensure that the tax is calculated on the total invoice amount, not on the individual line items.
* Save the tax.

Hope it helps

I think this such a non-standard way to record taxes because taxes apply on products/services not on invoice since invoice may include multiple products with different taxes and rates. It's also needed to make credit notes keep with tax applies on specific product refunded. 

If it's a matter of time needed to enter taxes for each product, Odoo can help by setting Default Purchase Tax (e.g. Tax 10%) for whole system (Invoicing > Settings) or per product (Product > Purchase > Vendor Taxes). So, all corresponding taxes will be loaded in product lines upon creation of vendor bill and whole tax amount will be also automatically calculated. 

https://www.odoo.com/documentation/16.0/applications/finance/accounting/taxes.html