At least one airline (Delta) changes a bonus fee to make a voluntary change to a ticket booked through a third party agency ($50). Unless the change can be made online or via the same-day confirmed travel program, you get that $50 on top of any other applicable fees.

No thanks.

The only time I use an OTA is when I have to combine multiple airlines and there's no other way to do it.

Edit: Someone was mentioning in the United forum today that they have a similar $25 fee.