You can set the default Tax and Surcharges for generating invoices using the the "TimeTracker Flags".
To set the default Tax or Surcharges
- Click the TimeTracker Flags tab in Salesforce
- Search for "invoice-header-tax-rate", enter the Tax rate in Number Value, and Save it.
- You can add the surcharge rate using the "invoice-header-surcharge-rate" flag.
The rate that is added in the Invoice header tax rate and invoice header surcharge rate is automatically added to the invoices that gets generated in your org.