Is it possible to make it so the buyer CAN NOT add an extra after the purchase has been made?

[kelarys]

I finally buckled and started offering ghost writing as an extra in my gig, after someone asked specifically if I do it (even though it stated in my FAQ that I absolutely do not) and I wanted the job so I added it. What I was completely unaware of was the fact that you could add an extra after an order was delivered. This was done to a piece of music that I was delighted with, and would never be willing to sell my rights to, especially as it was never discussed. I’ve made a note now in my FAQ that if you want that extra you have to commit to it right from the start, but I would like if at all possible to actually lock the feature once the order has been made. I’m fine with it being the difference between getting the job and not, but I value getting my name out a lot more than the amount I’m charging to be “bought out”
Fortunately the customer is being understanding, otherwise I would have to open a dispute that I would probably lose, but now we have to go through the pain of requesting a refund for an extra through customer support (why isn’t this easier?)