I have received an invoice for the full amount, how do I get a discount?

It is correct that your invoice shows the full amount. The actual benefit is determined by the arrangement your employer has made, not by the invoice amount itself.

If you are using a gross-to-net benefit, we cannot determine this benefit for you — it depends on your hours and salary, which we do not have insight into. The calculation example you saw during your registration provides a good indication of this.