Train ticket fine

Hi guys, so i purchased a ticket from trainline which automatically selected 16-25 railcard even though i dont have one the app automatically selected that and i have received a 100 pound fine at the train station. Genuinely shocked and if i appeal i do not have evidence to prove otherwise. Anyone dealt with a situation like this before?

Was it a fine, or was it a penalty fare? (They're not the same thing.)

It was a penalty fare, do i have any legs to stand on or should i just pay it

I would just pay it, and make sure you double-check what you're buying next time.

According to https://www.nationalrail.co.uk/times_fares/ticket_types/penalty-fares.aspx the amount is reduced to £50 if you pay within 21 days.