Hey there, @charlyn.
How's your day going so far? I hope it's going great.
In QuickBooks Online, the only way to show an invoice as paid would be to record a payment for the invoice manually. I'll walk you through the process.
1. Click the +New button.
2. Choose Receive payment under the Customer's column.
3. In the Receive Payment screen, enter the customer name or select Find invoice no.
4. Fill out the rest of the form and hit Save and close.
Please let me know if you need more assistance. I'll keep a lookout for your post. You can always reach back out anytime you need help. Take care!
Thanks for your help. I posted this question from the iOS app, and was hoping for a solution there. However, I logged in on the desktop and was able to accomplish it as per your instructions. Thanks again.