Need polite response to "invitation" to paint for free

Just got off the phone with a PTA member for a local elementary school.  They have "invited" us to paint for their movie night.  We could:
1) Paint for free and give out our business cards to promote our business.  I can't believe people still think that's such a great deal they're offering...giving us EXPOSURE.  Don't they know you can die from exposure?  
2) We can charge per face and then give all the money back to the PTA.  Don't get me started.  

I explained that we actually offer schools a 20% discount off our normal rate. (I work for the school system, by the way) and other area schools do pay our rate less 20%.

And I should tell you, the school is in a very well-to-do area.  Not 1-percenters rich, but families making probably 125-150K+ a year, houses run about 600-800K, and driving around in a Lexus or Mercedes...or both.  Meanwhile, I make about 31K working for the school system and end up putting in an extra couple hours unpaid every day.  I paint to supplement my income.

I will always volunteer my time for police and fire department events and a few other events like "Night to Shine" (Tim Tebow's prom for Special Needs teens.  I used to drive a Special Needs school bus.)

What I need are words to say that won't sound like I'm stumbling over myself because frankly I'm gobsmacked to think they would suggest the idea.

I'm sure this topic has been discussed ad nauseam, but I would appreciate any input or just point me in the direction of other posts.

Gotta calm down.    Thanks, all.

Check out Gretchen's blog with some good tips:
http://paintertainment.blogspot.com/2017/06/so-youve-been-asked-to-work-for-exposure.html

or this one from Ruby Red:
http://www.rubyredpaint.com/faq/faqexposure.htm

and this is good too:
http://ladydeelg.com/2015/01/10-things-professional-facepainter-wants-know/

I bet they had to pay to rent the movie, and any supplies needed. ditto if are offering free food.

if it is being held at the school and a staff member is required to be on hand, they are probably getting paid their salary, or possibly overtime for the time.

you are self employed as a face painter and it is your business that you rely on for income. as the articles kris mentioned say - you have already delegated your charitable budget for the organizations you mentioned.

learning to face paint takes at least several years of practice, similar to at minimum a 2 yr college associates degree. and it usually takes much longer, more like a bachelors or graduate degree.