According to [their transport information][1]:

> You may take as your hand luggage at no additional charge objects
> which are easy to move – those which may be freely placed above and
> under the space occupied by you in train. 
> 
> **When I have to pay for my luggage?**  
> 

> - If you transport objects which are
   > easy to move but cannot be freely placed above and under the space
   > occupied by you on train. In that event an extra fee of PLN 5.10 is
   > charged for each oversize item;  
> -  When transporting one unfolded and
   > unpackaged bicycle (details and instructions on the transport of
   > bicycles can be found in our "Guide for bicycle owners").

If you don't pay beforehand:

> In the event that a passenger has not paid a fare at a ticket window
> for the transport of an item, the intention that this be transported
> should be reported to the conductor prior to the journey beginning,
> i.e. before entering or immediately after entering the train. A ticket
> for the transport of an item on an EIP train may be purchased at a
> ticket window only.
> 
> Please be reminded that an additional charge of PLN 10 is made for the
> issue of a ticket from a conductor on a train.

If you fail to do that, then fines are incurred:

> In the event that a passenger has not forewarned the conductor of the
> intention that an item be transported, then – besides the charge for
> item transport of PLN 5.10 – an additional charge is also made, set on
> the basis of the directive of the Ministry of Infrastructure of 20
> January 2005. The charge at this point amounts to PLN 52, and in the
> event that settlement of the amount due is refused a summons is issued
> – a protocol with an additional charge of PLN 260. If the passenger
> pays the charge within seven days of the date of issue of the summons,
> the charge amounts to PLN 65.

  [1]: http://www.intercity.pl/en/site/for-passengers/services/transport-of-bicycles,-items-and-pets/przewoz-rzeczy-en/