Du kannst hier auch auf /de/ posten-wird oft genug so getan.

These have to be done at a PKP-counter and there its free, thus while you arr. in Szcz- it is very, very common in PL to do that just before boarding-though at weekends they may tell you its full-Polski needing to travel still board. Check also if there is not a way via PolRegio (=PL nahverkehr). In fact it may even be quicker to do Sczc -Gdy via Poznan, the line along the coast is not scenic and very slow and hardly had through trains some time ago.

I saw there is apparently now a method to do it online on the PKP-intercity site (which is often hacked or not properly working lately), but it looked rather complicated to me-and no, in any case, the DE way of SoFort or other bank will never work for foreign transactions and foreign providers.

Note that trains in PL get very full TO their coast on start of holidays=early july and BACK from there at end (quite logisch, nah?) and a little less to the mountains in the south of Krakow (Zakopane/Tatra) same times.