First you plan a vacation. Then you get an email for a VERY large order. Well you figure out what you'd normally charge, then up it to what you should charge (plus a little more.. ) an email them back... Then kick yourself in the ass when the accept it and you have to postpone your vaca!!! Easy peasy!
I think I'd better start planning more vacations!
I have always done this! Every time any asks me to do anything I don't want to do, I double my fee. If they hire me anyway - it's worth it!