You've already charged for the photos, you can't charge again, that's unethical. The truth is that, subject to your agreement, they own the photos. If they ask you to enhance those photos by all means charge them for the work.
Warbler, in the majority of cases with commercial photography (weddings excluded) you are selling your photos with the copyright. Newspapers, magazines, govt departments, commercial enterprises would all expect to employ you for your skills and the emphasis is on "employ". These larger organisations simply have a task to be done for which you are an employee or contractor, they own the product by choice, or otherwise they employ someone else.
Plenty of professional photographers out there willing to work without worrying about amateurs unless they want a giveaway once up price or don't have the budget. Either way, repeat business is unlikely.
Also, photography is a negotiated business based on what and when and rarely has anything to do with "going rates".