The devil is all in the details. If the customer has been a PIA all along and this change came right when the original spec was all designed up and ready to deliver maybe he was adding the fee on to discourage any more changes so he could get the final check and not work with this person any more. If you've got a contract that pays out your profit margin at the end of the job, you want to get it all tied up in ribbons and bows so you can pay the rent.
I've had jobs where I would swear the customer was dragging their feet and wanting revs or more documentation because they didn't want to pay or couldn't. They may have also found a competing product going for so little that they knew they could never get any sales and wanted to chase the designer off without having to pay them anymore. That happens a lot with crowd-funded projects. Lots of Asian company troll those sites looking for good ideas and can have a slap dash version of the product out before the funding drive ends. I saw one where they even ripped off the photos from the Kickstarter page.