PolishTrains is some dodgy third party reseller, ignore them. You should always book direct.
One option is to take one of the trains jointly operated by ČD and PKP IC, these can be booked at cd.cz or intercity.pl. PKP IC only starts presale 30 days in advance but I believe ČD opens earlier (if not, book those 30 days before travel for the best price). Booking with ČD gives you better flexibility for the cheap fares, otherwise book with whoever's cheaper.
Another option is Leo Express, which can be booked at le.cz. If you choose them, book one of the higher travel classes. You don't want to be stuck in their economy for such a long trip.