I do business with a Chinese company ran by my cousin. And each year I have to pay it about 30,000 dollars for marketing fees. Since the company is a small one-member firm, they do not have formal invoice. They print out the invoice for me in a letter sized paper, with NO title page, but my relative signed it at the bottom. Will that cause the IRS audit? If so, how they will verify a Chinese company invoice?

You can and should claim the deduction on your tax return.

If the IRS challenges it, then you may have to prove that the company exists in China, which should not be too difficult if your cousin is paying Chinese taxes on the $30,000 you are paying him.

Thank you AtlantaTaxExpert!
To prove that his company exists in China is easy, since I have the copy of his firm's certificate of formation. And he did pay tax every year to Chinese Government. I can show IRS my cousin's company annual tax report to Chinese Government. But I won't be able to show the specific tax amt paid on my 30,000 dollars. Is that OK?

Yes, that is okay, because what he paid to the Chinese government in taxes IN NO WAY affects the deductibility of the $30K payment you made to his company.