In fact, there is no actual invoice on a Discogs order. The status Invoice Sent simply means that the seller has set the shipping costs and the order is ready to be paid. Payment is made separately, whether via PayPal, bank transfer, or any of our other various available payment methods. As such, there is no invoice that is associated with an order page.
If you are a seller and wish to access your sales fee invoices, that can be done from the billing page.