Your invoices should be records in their own table, probably linked by customer id. You would be able to repeatedly reproduce a replica of the report based on month, customer id or any other criteria you wish to apply. If you have a price/costs table to feed those numbers into your invoice records, that means you have to watch out how you use those numbers. One way is to store the costs/discounts in the invoice record rather than link to those numbers. If you link to the numbers and later increase them, those reports would reflect the new values, which would be historically inaccurate. Consider posting a pic of your table relationships if you've created them (you can't copy/paste images here so see How to attach files at the top of the page). Be prepared for questions though, since understanding the process is crucial to giving any design advice.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.