Honestly, it might be an idea to contact itch.io support about this, as the site UI certainly suggests paying the shown price is possible, and I'm guessing you didn't get a warning at your end that users wouldn't be able to pay the displayed price.
Viewing post in Sale's kind of misleading (but it's itch.io's fault, probably)
You're right. There was no warning that I had noticed, but, I imagine it would be written somewhere on the site or the terms though.
I believe/think? the reason that the site permits it, is because the developer payout is still what is being advertised, while the itch.io fee is sort of a tax on top and is being displayed as its own separate warning before the player purchases the game.