Booking with Interrail is always more expensive as they do add a booking fee. Why not book directly? 

https://www.intercity.pl/en/

You can make domestic reservations on intercity.pl, following the instructions here:

https://www.seat61.com/interrail-and-eurail-reservations.htm#Poland

Reservations only open 30 days before travel. 

Instructions for international trains depend on the destination, details on the same page.

I follow the instructions. But i never see the option that i can choose i have a interrail. 

Did you see these instructions?

https://www.seat61.com/interrail-and-eurail-reservations.htm#How_to_make_reservation_at_the_Polish_Railways_website

Hi, I don't know if this could help but I contacted PKP customer service a few days ago because even if I followed the procedere the Interrail discount didn't appear and this is the answer I received:
"On behalf of PKP Intercity S.A. thank you very much for your correspondence.

I would like to kindly inform you that for the Interrail ticket you can book a seat through https://www.interrail.eu/en or you have to go to the international ticket office at the train station."

It's indeed possible to book with them. Here you do see a screenshot from PKP IC's booking page with Interrail as discount: