How do you include outstanding balances on a current invoice and have the total due on the pay link = the outstanding plus current balance

I'll share details about the invoice payment links in QuickBooks Online (QBO) along with the other way around you can use, Michael. 

In QBO, each payment link is unique for every invoice transaction you generate for the customer. The option to include those outstanding balances of the previous invoice in the current one is unavailable. That said, they have to pay the invoice individually using the specific links provided. For a visual reference, feel free to check this article: Take and process payments in QuickBooks Online with QuickBooks Payments.

Furthermore, payment links will help your customers easily determine those outstanding invoices they have. To see this, they need to click the "You have [# of invoice transaction] unpaid invoices" drop-down that is located at the upper-right part of the payment page. The View hyperlink inline with the invoices will route them to the payment page where they can settle the balance. 

On the other hand, you can generate a statement for them that lists all their outstanding balances. If they want to pay the total amount, you can manually record this instead as a deposit and apply it to those unpaid invoices. Here's how:

1. Select the +New menu and choose Statement.
2. Choose Balance Forward in the Statement Type box and enter the correct date in the Statement Date box.
3. Select Open or Overdue in the Customer Balance Status field and input the Start Date and End Date.
4. Next, in the Recipients Lists section, select the customers you want to create a statement and make sure their email address is correctly entered.
5. When done, click Save and send.

You can also modify your invoice layout to include details necessary for your company. For a detailed guide, refer to this link: Customize invoices, estimates, and sales receipts in QuickBooks Online.

For other invoice questions you may have, please know that you're always welcome to post here again. I'll make sure everything is sorted out so you can get back on track. Have a great day ahead.