Remax manages the property that my roommate and I rent. After signing a renewal lease (for one year), my roommate was sent an email stating we now owe a renewal lease fee of $75.
This fee was not incorporated in the lease when we signed it or submitted it.

Is this legal?

Is it clearly posted in their office?

I wouldn't know. We do all dealings electronically per their policy. I've never even been to their office.

They might consider it an administration fee... which would not be on the lease... if it's a one time charge.

Unless the fee was communicated to you beforehand, whether in the lease, fine print in their e-mail, or whatever, no, they are not entitled to charge the fee. Ask them how they figure they communicated this to you.