Anyone tried this service who can indicate how you go about it please?
Easy. Make sure that the envelope is addressed with the name of the recipient at the other end, and give them 30zł. You need to be quick (if it's a mid station) - but normally there's enough time to find the conductor and get them to take it. The person on the other end will need their ID to collect the envelope, too. The other thing is to make sure that you have exactly 30zł for the fee, because they might not have time to fool around looking for change.
It's all here - intercity.pl/pl/site/dla-pasazera/3052/przesylki-konduktorskie.html
If you can be bothered, there's a stationary point in Gdynia where you can leave it so you don't need the hassle of meeting the train, and it's 11zł extra for that service.