Send monthly bill for a lot of smaller things?

I have sn idea of a web service where I will take a provision of things that my customers are selling. My customers’ orders will pass through my website via an API. And here is where I have my chance to see the cost for those orders and add my provision fee that will later be billed my customer in a ”united” invoice for all the provisions that month.

So as I said, it will be invoiced every month. So the question here is what I will do when I add that provision fees all the times during the month. Should it be added to like an open order? Is that possible to have? If so, how? Should every fee be added like a product to that order? Or is it possible to have just one product named i.e. ”Provision” and then spec all the provision fees for all the orders my customer had last month on separate rows below?

What is the way to go?