you do not have to select every customer to apply discounts as default for them.
the sales invoices settings in Form Defaults works just like the way when you create a sales invoice, i.e. you have to select a customer to make the bottom portion of the form to appear.
when setting the sales invoice defaults just select any customer, set the discount you need and remove the customer before you click Update.
now the discount will appear as a default for all sales invoices irrespective of the customer selected.
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