Sign a deal memo outlining the exact payment for the shoot. The deal memo should have on it the credit card number of the person who is paying. Before the shoot call the credit card company to verify the card is good. After the shoot, simply submit the card number for payment. If the producer balks then you have the deal memo to back up your side of the story.
Drawbacks are a bit more delay in payment and a credit card surcharge as a vendor.
This is to confirm that you, acting under your company authorization, have retained my services, as the Director of Photography, on the above mentioned project as per our telephone conversation. The production company will provide 1st or Business Class Air Transportation, and is responsible for Hotel room and taxes, and will provide automobile transportation while on location. I will bill by invoice and payment is due upon completion of photography. A Beta SP of the completed project will be provided to me. Please sign below and FAX back to acknowledge receipt of this deal memo.