I've been working on a long term project with multiple alternate versions. Part way through the project, my client asked if I could add more versions, which I agreed to for a fee. At first, I didn't charge them for one of the versions since it was just a combination of two of the other versions (so I wouldn't have to do any actual drawing for that version).
I didn't start on these extra versions for awhile, so I actually forgot I didn't charge them for that one.
Fastforward a bit, and I send off the roughs of the alternate versions for approval to start them. The client asks for some changes which weren't charged for as they fall under the first couple changes being free that I lay out in my TOS.
However. At the time, it didn't occur to me that one of the changes would mean I would have to paint something on that version that I had originally not charged for. I should have asked for the payment for that one right then, but I ended up completing that version before I realized what happened.
To be clear, I have received full payment for both the original project and the addition of the extra versions. The project isn't over yet, and I haven't sent anything but small watermarked files, but would I be right in thinking I can't ask for compensation now and should just eat the cost? It isn't a lot of money, but it could maybe happen again and so I'd like to know people's thoughts.
Sorry if I described it in confusingly vague terms, I don't want to seem like I'm pointing a finger at the commissioner because they are an excellent customer.