I know you get an email for them, but is there place on the paypal.com website to see what invoices you have received, the only place to see them is via email?
From your PayPal Summary or Activity page, use the "Classic Site" link at the bottom of the page to view your History page, and then select "Money requests" from the drop-down "Filters" menu. Be sure you have the correct date range entered... this may take two tries because sometimes the selected date range reverts back to the previous setting.
