Tip: Make sure you know the order of what you want to say, in general, in order; the write what you want to say (almost a memorized script); then create your slides based on a template. This approach has worked for me. I hope it works for you.

I’d tell the client that you made a mistake, that you are going to honor your price, but that future work will be priced independently (i.e., the current price is a one time deal). Or, don’t even tell them and just live with the price you negotiated. The burden for this mistake should be carried by you, not the customer. The upside in telling them it was a mistake is that they may open the door to re-negotiation. But let them bring up the idea. Regardless, under promise and over deliver.