All I do are custom orders, mostly party invites and other favors. I keep my prices high so that all of my orders include any changes they want that is within my power. This keeps my buyers very happy!
I have my files set up so I can easily change most colors and of course wording, fonts and wording color are always simple to change, so I offer it free of charge!
It's the way I do it, and it works for me. I have a hard time asking for more money after the original transactions, but it might not be a problem for you. I say do what feels right to you. At the end of the day, it's your store and you can pick your policies.