I pay multiple vendors and use the option to save the checks to print later as a batch. How can I add an additional invoice to be paid to a check not yet printed?

Thank you for bringing this out, @eblake.

To add the old invoice amount to the existing payment, you need to delete the said payment first. To do that, follow these steps:

1. Open the existing payment.
2. Select the More option.
3. Choose Delete.

Once done, you can record the old invoice amount. I'm glad to show you how:

1. Select the + New button.
2. Select Bill.
3. Enter the needed information.

Lastly, you can now make payments by using the Pay bills feature. Make sure to select the entries associated with the vendors. This way all amounts are included or incorporated in the Print Checks window.

When you're to print your checks, you can use this article as a reference: Print a check in QuickBooks Online.

I've also included these resources to help you categorize your transactions while making sure every figure is balanced and accurate:
 

• Categorize online bank transactions in QuickBooks Online
• Reconcile an account in QuickBooks Online

If you have any questions or need further assistance about managing your vendor transactions, feel free to visit this page and add comments. I'm here to help. Have a great day!