The 20% rebate is automatically calculated in the amount of coin requested from Coinpayments to complete the transaction. When sending payment you will see the amount requested reflects a 20% discount plus a small transaction fee. This is automatically built in and nothing needs to be done.
The rebate is not calculated in the order subtotal on the Mitragaia website so you will see the full price in the order total. You won't see a discount until sending payment from your personal wallet.
Let us know if you have any further questions.