How do i mark an invoice paid?

Welcome to the Community, @dbonner.

To mark an invoice as paid, you'll need to record the payment against that invoice. Let me show you how: 

1. Go to the Sales tab on the left menu.
2. Click Invoices.
3. Locate the invoice from the list you want to mark as paid.
4. Click the Receive payment under the Actions column.
5. Enter the payment method and any other necessary information.
6. Select Save and close.

For more information, you can read through our help article: Record a customer payment. It'll also guide and help you learn about how to record partial or joint payments. 

I'd be happy to know how it goes, @dbonner. We're always here to back you up. Have a great day ahead!