Hi there, @Rosieb. Let me guide you in marking your invoice as paid in QuickBooks Online (QBO).

In QBO, you can mark an invoice as paid without a connected bank account unless you're making a deposit.  In your case, you can use the Receive payment option to keep track of your customer's balances.

Here are the steps to follow:

1. Navigate to the All Apps tab.
2. Select Sales & Get Paid, then click Invoices.
3. Locate the invoice you want to mark as paid, then click Receive payment.
4. Enter the customer’s name in the Customer field and complete any required details.
5. Check the box for the invoice you want to record.
6. Save and Close once done.

If you need further assistance, feel free to reply to this thread. We're here to help.