Either you are confident and assertive or you are not. Individuals, especially business owners, can sense when we are not. I don't think putting up a sign is going to help your situation. If you lack the confidence in obtaining payment, how will you respond when these individuals disregard the sign? Seems as though you will have the same result. In addition, if these individuals are business owners themselves, they will understand. My advice is to sit down and brainstorm a tactful sentence or two in response to this situation. Ask others for help if you are stumped. Once you think you have one, practice the response in the mirror, say it over and over to yourself throughout the day until it is etched in your brain and sounds natural. Sounds corny, but you will project confidence and not have to rely on ad lib responses. So this is one of your weaknesses. Nobody is perfect. Strengthen your weakness. Lastly, after you ask for payment with your practiced response, don't say anything more. Let them have the next word, even if there is a length of uncomfortable silence.