It's generally not a good idea to order reservations to be sent by post, unless you have a lot of time left. Sending them to your hotel/hostel is not a good idea. If you order via Interrail, they'll send them via registered post and someone needs to sign for them or pick it up somewhere. How that works depends on the country/postal service and you first need to agree with your host that they can do that for you. We've seen too many stories of this going wrong. And you pay a premium price for it because you pay an extra €9 delivery fee per order on top of the already €2 booking fee per seat.

But there are often more ways to book. What train(s) do you want to book? Route, date, departure time.

Domestic trains in Poland can be booked on intercity.pl. For other trains, it depends.

Thank you very much for your answer, as I was assuming something like this, I looked at the Intercity page in Poland and there were all the trains. Once reserved there, do you have to print the ticket on paper?thank you very much

You don't have to print your seats reservations. It's enough to show them on a screen.