Hey there, @HeatherG2.

How are you doing today? I hope all is well. 

At this time, there isn't a way to add the balance of a missed invoice to the new one. I understand how beneficial this option would be for you and your business. With that said, I think this would be the perfect opportunity to send this suggestion to our Product Development Team. Our developers review each request and consider them all for future updates. You can use the link I'm including below to send your feedback anytime. 

• Send Feedback in QuickBooks Online

On the other hand, you could send your customer a balance forward statement. A statement will show a complete transaction history. You can reference my screenshot example below. 

If this is something you're interested in, I'm including some steps to walk you through the process of creating a statement below. 

Here's how: 

1. Go to Customers.
2. Find the customer on the list.
3. In the action column, select the Action menu icon, then Create statement.
4. From the Statement Type dropdown menu, select Balance Forward.
5. Enter the Statement Date, Start Date, and End Date.
6. Tap on Print or Preview or Save and Send.

To add the details for each transaction in the statement:

1. Click on the Gear Icon and choose Account and Settings.
2. Select Sales.
3. In the Statements section, tap on Edit.
4. Select List each transaction including all detail lines.
5. Hit Save, then Done.

Another option you can try is customizing an old invoice layout and including the customer's account summary. The account summary summarizes your customer's info from the last year. It includes:

• Balance Forward: The total amount due and any credit balance from the last invoice.
• Payments and credits: All payments or credits since the last invoice.
• New charges: The total amount of new charges and credits on the current invoice.
• Total amount due: The customer's total balance.

For more information and steps to complete this, check out Add an Account Summary to an Invoice in QuickBooks Online. This guide also includes more details about statements. 

Please let me know if there is anything else I can assist you with. Take care and have a wonderful day!