For buses inside Germany, and not only, check Meinferbus/Flixbus (unfortunately they don't have routes to Poland cities (yet)). For example, from Berlin to Frankfurt am Main the price would be 22 Euro.
So, Option 1: Szczecin-Berlin ; Berling - Frankfurt by Meinferbus/Flixbus.
Another option (much better I think), there are very cheap flights from Warsaw Modlin to Bonn/Cologne Airport via Ryanair, 9.99 Euros (not every flight is this price but you can find it always in 2 weeks range), and from Cologne, to be more specific, from Leverkusen (which is apprx. 20 minutes away by local train from the Airport) you have a bus connection, again with Meinferbus/Flixbus for 9 Euro. In total it would make, say, 20 euros for you, from Warsaw to Frankfurt.
So, if you can travel so easily inside Poland, Warsaw should not be a problem, and it will be quite a fast travel, Leverkusen-Frankfurt takes only 2 hours and 45 minutes by bus.