It happened. Your cousin needs that website for their auto shop. "Cool," you said. "It will be easy," you said. After all, it's just a brochure site, with a contact form, an "about" page, and a short description of the kinds of cars they specialize in, right?
One absorbing weekend later, you're putting the finishing touches on it ... when your cousin calls and says that he talked to the boss, and the boss thinks that it would be "really great" if they could also sell some stuff on the site, like motor oil and anti-freeze. "Just simple stuff, right? Thanks. Oh ... and could you also set up a scheduling system so that customers can make appointments online?"
When I was younger, someone told me that I should say yes to everything, and then figure it out later. That's still true ... ish.
There are a lot more reasons to say no than there are to say yes, and it's for your own protection and sanity. And if you were smart about it, you already got half your money up front.
I love "Yes, and." Most people know it as an improv comedy rule, but it also works for the kind of soft-skill client wrangling that we all have to get good at, if we want anyone to speak to us again. Has your client asked you to basically recreate the Facebook website for them? That's a "Yes, and" ... even though it's clearly a no.
Let me explain: People don't really like hearing the word "No," even if they understand the reasoning behind it. They feel the negative, even if it is 100% correct. So when you're saying "No," you could also say something like:
I remember reading somewhere that the words that are used matter, greatly. So if the phrasing is positive and open, you're more likely to get a good result, even if you're technically turning this person down. It matters even more if you think that this contact would lead you to more work in the future. It's the difference between "No" and "I can't help you with that right now, but I'm open to other stuff in the future!"
Don't fall for the oft-repeated notion, "The customer is always right." You need to get that notion out of your head. Otherwise, why are you starting a second career? I thought you wanted to be your own boss? Part of being your own boss is being able to set your own boundaries. Define those, and you'll be avoiding a lot of time-consuming, confusing situations.