Did you do something naughty to slrpnk.net by any chance? I only saw your comment incidentally when viewing my thread directly on the custodial host. I cannot even force your comment to appear by searching the URL on slrpnk.net. I am not blocking feddit.org either.
Anyway, not sure you will see my reply because I cannot use the reply button on your post; but I’ll answer here:
Source? You’re definitely not paying 4x the online price when you pay in cash for a bus ticket where I live.
I’m not sure if this Flixbus problem is published anywhere. I can only speak from experience. A ticket starting at €10 will often get as high as €40—60 at the moment of departure. You can verify this just by looking at buses departing today and comparing to the same route a couple months into the future.
(edit) I imagine you are thinking in terms of public transport. In that case, some bus networks outright refuse cash at all possible sales points including drivers. There are some press shops that sell tickets for cash but only as a 10 pack, not single trips. So a cash payer who needs a simple one-off to go to the airport is fucked.