Hi there, @AppleLa2. I got your back in marking the invoice as paid in QuickBooks Online (QBO).

You'll have to use the Receive Payment feature and enter the invoice full amount to mark it as paid. Let me guide you how:

1. Sign in to your QBO account.
2. Then, click the Sales menu and select Customers.
3. Look for the customer where the invoice is allocated. Click the name to open.
4. Next, find the invoice you want to mark as paid and select the Receive payment button.
5. In the Amount received field, input the full amount received for the invoice.
6. Once done, click Save and close. 

For more information on marking the invoice as paid in QBO, feel free to visit this article: Record invoice payments in QuickBooks Online. 

Getting a list of your customers’ outstanding balances will help you determine how much still due and how long they’re past due are. You just simply need to run the accounts receivable aging report to have this data. 

I'm always around if you need additional assistance in marking the invoice as paid in QBO. Don't hesitate to post your concern here. Have a good one.