If I follow that line of logic to its final conclusion, shouldn't I just work for free all the time then?
I don't think you understood.
The boss sounds like they are looking for a free ticket.
You then offer them $400 of work for $400.
Not what the boss wanted as they probably don't need $400 worth of pictures for their purposes (unless an offer they can't refuse such as for free).
It's a small, private clinic. Social interactions can have a big impact on your job and happiness within that job. The OP's language already indicates some underlying tension between employee and boss (but where is there ever not).
It would be completely different if you worked for a big company.
Is it worth the potential aggravation for the sake of $400?
I'm not saying you should work for free, no way. But if you're not willing to do
this favour for free as implied by the boss then walk away and get business from somewhere you can have a direct client relationship with.
Saying all this, it's tricky to judge situations across the internet with only a snippet of information provided.
Only the OP knows everything in its entirety, including the personalities of everyone involved and how they react / will react. It should absolutely be their call at the end of the day.
Either way talk with the boss and make sure you're both completely aware of what the plan is i.e. don't do the pictures and
then slap a surprise invoice on their desk that they weren't expecting. But I hope that goes without saying this time...