In the overview that opens by clicking Invoices, you can see all your invoices created via Instant Permissions. If the Is Paid column says Yes, you have already paid the invoice. All invoices that are still open are set to No.
Click on the invoice number (in the Invoice Number column) to open another page. Here you can download the invoice as PDF (Download as PDF) or open it in the browser (Preview Invoice).
Alternatively, you can download an invoice by navigating to the Offers page and then My Permission Offers. Go to the Awaiting Payment or Completed tab and then click on the blue View button of the permit you were looking for. On the summary page that opens, you can download your invoice (click on "Pay by Invoice"), conveniently pay the outstanding balance by credit card (click on "Pay by Credit Card"), and download the entire contract (click on "Download Contract").