The sending of a bill now (which is generally an itemized statement of the actual amount of costs owed by one to another) does not
preclude the sending of a settlement demand letter later (which generally details the infringement, states a specific dollar amount to be paid often based on the amount of statutory damages that could be awarded, and will threaten legal action if it is not paid).
If both a bill that is sent and any subsequent demand letter that is sent are ignored by the business, photo guy still has the option of suing the business for copyright infringement. He does not have to go the collections route.
Both a bill and a demand letter, in other words, are designed to accomplish the same goal - to get the business to compensate photo guy for the use of his photos on the business website. Either could put an end to any dispute over the illegal use of the photos, if the dollar amount photo guy is asking is a reasonable one and the business pays that amount.
I guess it is always possible, too, that photo guy is using the word "bill" when what he actually sent was structured more like a demand letter.
Oops. I see that you posted, photo guy, while I was composing my post. Hopefully the business will pay what you are demanding and the matter can end for you there.
As for thoughts on copyright infringement on the internet, unfortunately there has been no good way to prevent it yet. Many people still believe that if material is found on the internet, it is free for anyone to use - or they figure they won't get caught so they go ahead and use it anyway. Right now all a copyright holder can do is monitor the internet for illegal uses of their copyrighted material, file DMCA takedown notices when necessary, and threaten to sue, or actually sue for infringement if it comes to that. It is good to hear that the Professional Photographer's Association will work with freelancers like yourself to handle infringement of their photos when it occurs.