That's an interesting one. I believe
TOCs▸ reserve the right to make a charge for seat reservations if made
after the original purchase of tickets.
Whether you are charged or not is down to various factors:
How that clerk is feeling on the day.
His/her directions given by head office.
How you ask.
Whether it's a Tuesday, or a month with an 'r' in the name.
Okay, I'm making up possible reasons, but.... In short, no definitive answer.