How do I mark an invoice as paid?

I've got the steps to mark an invoice as paid in the program, @sonoftoil. I'll guide you how. 

1. Go to Sales from the left menu. 
2. Select Invoices. 
3. Locate the invoice.
4. Click Receive payment from the Actions column.
5. Verify the information in the Receive Payment screen. 
6. Hit Save and close. 

The screenshots below show you the steps. 

Once done, the invoice's status will be marked as Paid from the All Sales menu. See the screenshot below for your visual reference. 

Also, you can use the Receive Payment feature from the + New menu. This is the same process I've provided above on how to receive an invoice payment. For more details, check out this article: Record Invoice Payments.

As always, you can run the Invoices and Received Payments report. This lets you view all the payments you’ve received and the invoices associated with them for a specific period. Just go to Reports from the left menu. Then, enter the report name in the Search box as seen in the screenshot below. 

Please know that I'm here anytime you have other concerns. Have a great rest of your day, @sonoftoil.