There won't be an expensive ending. You've seen the price and that's what you pay. There are usually three prices, the 'Standard' which is carry on only, no priority boarding and no choice of seat, 'Plus' which gives you 20kg of checked luggage, priority boarding and seat reservation and finally 'Flexi Plus' which is the same as 'Plus' but with flexible tickets, fast track and airport check in. Personally I don't see the value in Flexi Plus considering it is typically double the price of 'Plus'.
As you complete the booking you're usually offered the opportunity to purchase travel insurance or car hire etc but there's nothing hidden or deceitful about this, you either accept or decline it.
Ryanair are providing a service that suits a variety of traveller. Some are happy with just a carry on, getting on the plane last and sitting anywhere because quite often the price of that ticket is £9.99 (as is the case from London to Gdansk). Others prefer to check their luggage as they like/need the extra storage and prefer the convenience of priority boarding and seat selection but this costs £31 for the same Gdansk flight which is still a fantastic price and one I'm happy to pay because it suits my requirements.
I've never booked with Ryanair and then discovered I'd been stung by some hidden fee, why would they do that? It's not exactly a good business model.
Is Gdansk a destination for you in itself or merely a transfer as Ryanair fly direct from Stansted to Rzeszow?