Extra luggage ticket on Intercity.pl necessary?

I was about to buy a ticket on intercity.pl and noticed that there is an option to select an "extra luggage" ticket for a few zloty.

• When do I need this ticket?

or:

• How much luggage can I carry on a regular ticket without the extra luggage option?

I will be traveling with a medium sized suitcase and a medium/large 55l backpack plus a small bag at most.

Bonus questions:

• Is this extra luggage ticket enforced if needed and one fails to purchase?
• Can one easily purchase the extra luggage ticket later on, or even on the train, without an extra fee? In my scenario I would buy an online ticket now, a few weeks before travel.

From what I noted the luggage option is available for all of the faster train types.