An invoice is compulsorily issued for every payment request.
Once it has been issued after a successfully completed payment request, it is immediately sent to the e-mail given in the user's profile.
You can view your invoices from the Completed payments page, by opening the detail of a payment and clicking, if it is there, on the link Download invoice.
Important: it is possible that, after clicking on Download invoice, the Completed payments page stops working once you have downloaded and opened the invoice. To repristinate the features of the page, just click on the Refresh button of your browser or press F5.