From a content perspective
Last time I had a photoshoot, I asked the company to flip some pictures. They firmly refused because it would not be 'good'.
I don't know the rationale other than 'it looks less natural', but I guess it is safe to say that flipping would be a compromise on quality.
From a business perspective
This brings us to the business question.
- Did you do what they asked?
- Did you do deliver something that someone without context would consider reasonable?
- Did you read their mind about what the context would be?
Assuming you did what they asked, delivered something reasonable and had no clue that they had hidden additional requirements, now this is the situation:
You can ask them whether they would like to flip the pictures, or whether they would like to hire you for a followup shoot.
Unless you have a long standing (and good) relationship with them, I would definitely not do a second shoot for free (discount or 'at cost' is of course possible). If you give this for free, you have a good chance that you poison all potential future profit from this customer because they will keep giving you extra work to do for free.