If you're about to pay in another currency than Swiss Francs, just know that when you make the payment, the conversion will be done automatically by your bank. So even if you have a debit/credit card with another currency, or a Paypal account with another currency, stay calm! It will work perfectly.
Just click on the red button noted "J'ai un bon de réduction" right below the Total amount, and your coupon will be applied!