This page allows you to see paid and refunded invoices. The search filter at the top of the page allows you to quickly find specific invoices by user's email, username or product name. Clicking on the table header links, such as "price" or "invoice date" will sort transactions by that column.
You may click on "details" link to see details of any particular invoice, including any related transactions.
You may click on "refund" if paid invoice has been refunded and you would like to add related transaction record to reflect it. Note that at this time software will not actually process a refund in your payment gateway for you, it will only make a record in its own database. You still have to process a refund in your payment gateway's account manually.