I want to be quite frank with my payment policy. Quite often I am asked to provide services for free, with a variety of potential justifications.
I have been a professional photographer for close to 40 years. I do not need to establish myself, I do not need work to create a portfolio. I have invested over $80,000 in equipment for my business. I pay my bills with the income I receive from photography. It is not a hobby. I do provide services at cost (as does their venue, caterer, etc) for a charity in which I believe. If you are looking to make money from services I provide, I expect to be paid for those services.
I think a good way to illustrate this is:
I called Canon, asked them for 3 5D Mark III's with an assortment of lenses and accesories for free. I told them I would tell everyone I saw how good their products were. They declined.
I called Adobe and asked for the Creative Suite 6 package including Photoshop, Premiere, etc. I told them I would tell everyone I saw how good their products were. They declined.
I called Apple and asked for a free MacBook Pro and iMac and a copy of Final Cut Pro to edit my movies. I told them I would tell everyone I saw how good their products were. They declined.
I tried the same thing with the grocery store, gas station, electric company, and mortgage company. All politely declined and one told me I should see a psychiatrist.
So these are some of the reasons why I have to charge for my services, and expect to be paid in a timely manner for those services. It doesn't matter to me if I am the only one involved in your production that is asking for payment. It's likely that I am the only one who has made a significant investment in equipment and experience.
Thank you for your understanding. I don't mean to be rude or insulting. It's just how business works.