So you go back to the post office and ask the clerk to get your letter so you can change the address.
I think I am going to try this with all of those Christmas cards I just dropped in the mail :) Seriously, this is a good suggestion. I just have to come up with an example that works in their industry, which might be hard for me to do.
Put it in your invoices: "added X feature (.5 hrs) no charge."
This is a great suggestion. I have heard this before, but I am not the best at remembering to actually do it. I will have to get better at that, especially with this client.