Select the standard charge model if you want to charge the same price for each unit consumed. Imagine that your API company charges $0.05 per API call (i.e. your billable metric). By selecting the standard charge model, you will define a fixed unit price. If a customer makes 1,000 API calls during the billing period, the total invoice amount will be $0.05 x 1,000 API calls = $50.
Only the standard & dynamic charge models support the custom invoice grouping.