On the website, it's 75p if using a debit card or Paypal (no idea about credit cards) and the iOS app charges 25p. There are no fees for tickets bought on the day of travel via the app.
Having now looked at Train Line's website, it's fairly easy to find the fees. The credit card fees are 2% on top of the transaction fees which are 75p for up to £35 and £1.50 above that. I am struggling to find any mention of the HBOS offer tho. What is meant by 'opt in'?