Financial Regressions

I hope you all had an amazing holiday weekend. Currently, I have a dataset that includes company name, company type, service provided, charge for the service, and this is all by month. For example, Company X, type of company is a bank, services provided are debt portfolio evaluations at $100 and data analytics at $3,500, and the month is November (obviously this is a fictional example). There would be two entries for this company because there are two services. What regression model would you recommend to calculate if a company is being under billed?
How many observations are in the data set?

How are you defining under-billed, including the time span this occurs over such as per service, monthly, quarterly...

Are you looking to see if an entire group (company type) is being under-billed on average, or only at specific companies?

These will be helpful for us to understand your project.
I have around 500 data points.

I am defining underbilling on a quarterly basis.

I want to do separate analyses where I check at the company type level and then I want to do an analysis at the company level.

Do you have any other ideas that maybe would be a better analysis based on this data?

Thanks so much!