I think the freelancer did much more than $12,500 worth of work for "Robin" over those two years. The duration and content of the whole project is told just as

I think the freelancer did much more than $12,500 worth of work for "Robin" over those two years. The duration and content of the whole project is told just as backstory, to illustrate that the freelancer had no reason to suspect "Robin" before this backcharge popped up. The $12,500 is just the amount the cardholder could backcharge -- as you say, in the last few months -- which is probably a rather small part of the whole.