Thats the correct way to do it,

Just make sure you've set the Maximum Discount to 0 for each of the product's Fixed and Unit prices.

Any new invoice with the products should have a zero discount.

There is one catch: if you edit an existing invoice which uses the products, the discount won't recalculate.

You need to manually set it to zero. This is because the application assumes that if the current discount is non-zero, and the calculated discount is zero, then it has been manually keyed, and so doesn't overwrite it with the calculated value.

-Tim