If an invoiced customer was a few dollars short on their payment, can I include the outstanding amount in their next monthly invoice, and if so, how?

Yes, you can include the customer's outstanding balance amount to their next monthly invoice, mpianos. This way, you'll be able to remind them of the total amount they owe to your business.

Recording a customer's partial payment on an invoice will automatically change its status to Partial when viewing it in your customer's Transaction List page. 

Based on your description of the issue above, it seems that you are sending a recurring invoice to your customer monthly. To include their outstanding amount, you can customize your recurring invoice template and add the open balance to the bottom. 

1. Go to the Gear icon.
2. Select Custom Form Styles.
3. Double-click the template name.
4. Select the Content tab.
5. Click the Footer section.
6. Add the balance details in the Message to customer or Add footer text field.
7. Click Done.

Once done, you can manually include the outstanding amount when sending your next monthly invoice.

In addition, you can also resend your customer's invoice as a reminder. This allows them to view their remaining balance, the breakdown of the transaction, and the applied partial payment.

1. Go to the Sales menu.
2. Choose Customers.
3. Select the name customer of the customer.
4. Click on the Transaction List tab.
5. Locate the invoice, then select Send Reminder from the Action column.
6. Click on Send from the pop-up.

Moreover, you can pull up the Sales by Customer Detail report to view the complete list of your customer's transactions and their payments in the Reports menu. Customise the report to get the specific details you want view. Then, memorize it to to save its current customisation settings. For the detailed steps, kindly refer to this article: Memorise reports in QuickBooks Online.

Please let me know if you have other concerns. I'm just around to help.