You can also, track your customer's transaction by using a Project feature. This feature is only available for QuickBooks Online (QBO) Plus, Advanced, and Accountant. If you are using Plus or higher version, here's how to use this feature:
Step 1: Enable Project Feature.
In your QBO company, go to the the Gear icon.
Select the Advanced tab on the left pane.
Choose Projects and tick the box that states Organize all job- related activity in one place.
Click Save, then select Done.
Step 2: Create a project.
Go to the Hamburger icon and select the Project menu.
Enter the name of the Project, the customer, then verify.
Step 3: Adding a transaction in the Project.
In your Project page, click the Add to project buttton. Check on the screenshot for reference.
Enter the details and Save.
You can check the invoice added in the Transactions button. I added a screens shot below for your reference.
If you are using lower version than QBO Plus, you can create an invoice transaction and then create a Transaction List of Customer report up to the end of the month to state the amount and the transactions to be billed for the customer. You can check this article in customizing report: Popular custom reports in QuickBooks Online.
Count me in if you have questions about the steps above. Happy Sunday!