I think that in the help section i saw 25 max was mentioned. Last time i mentioned 25 and it was fine and got the records without any charge. This time too i mentioned the same and the request is in process now.
Most requests are not that difficult i guess hence they dont charge. I dont know their criteria for fee assessment but here is what the website say.
Note on website:
"In the event that fees are incurred beyond what you are entitled to for free, please indicate a specific amount that you are willing to pay. If you do not indicate another amount in FOIAonline, please be advised that some agency regulations provide that making a FOIA request represents an agreement to pay fees up to $25 (See agency specific regulations from the links on the main page). "