If you paid the I-485 package deal after the fee increase last year which was I think $1010 or so, then your EAD renewal is free. But if you paid the old fee (where there was a seperate fee for eveything), even if you filed after July 31, then you still have to pay the $340 for renewal. I went thru the same thing not too long ago. I called USCIS and they said I do not owe anything, (blah ,blah, after July 31st), but when my lawyer sent in the papers, we found out that they have no clue what they are talking about when you call.
Hope this helps.