CORE is flexible regarding applying discounts to bill amounts before and after taxes. By default, it applies a discount after the taxes. 

Here are the steps to do so:

1. Open the Invoices screen and create an invoice. Check CORE Help Center for details on creating invoices.
2. Apply a discount to a billing record by entering an amount, which reduces the Net Bill amount after taxes have been added. It does not reduce the tax amount of either services or expenses. 
3. Process the invoice as usual. 

However, if you want to apply a discount to the bill amount before taxes, go to Settings > Billing & Invoices > General and select Apply discount to pre-tax amount.