This route starts at Vaasa, and runs via Turku and Helsinki, on a 28 hour overall journey, ending at Warsaw.
Fares start at around £54 for the entire journey, which includes the ferry fare from Helsinki to Tallinn (Estonia).
https://www.youtube.com/watch?v=9HbeZU76DYQ
But note that on the day the above video was filmed, the toilet on the bus was not working. Given the brevity of the stops Flixbus makes that was unfortunate, to say the least.