Interrail offers from time to time special discounts. You can get 5% to 15% on your pass purchase. Often these special promos are in spring (March or April) and at the end of the year in December. The good thing: you can buy your pass to this discount. And choose your departure date within 11 months. Usually these promo offers are not refundable.
I will inform you here, if a discount is available.