Thanks to all who have contributed, especially Krzysztof with his detailed reply. This was what I had been looking for.
I can see that it was really my luck to get the jeans refunded as 13 days had elapsed from purchase (I am not permanently based in Poland yet, and only had a chance to return this week).
As a comparison, I live in Lithuania, and the issue is regulated in a more favourable way to the consumer here. One can return or exchange most types of goods, without signs of use, within 14 days from the date of purchase. It is the customer who decides if the item must be exchanged or returned for a full refund.
In the case of faulty goods, it is again at the customer's discretion to choose repair, replacement or refund.
In Poland - you can buy one item in one store and return it in another I think :).
Try it and find out, but if you get arrested for deceit
Why would that amount to deceit? I am not sure it is legal, but it does look reasonable from a customer satisfaction point of view.