You can find draft invoices under the PO invoices tab of the Control Panel, in Reports center -> Accounting .
2Checkout includes information on Purchase Orders into your Sales Reports, both in the body of the email and in the attached files. You can get insight into all approved Purchase Orders, as well as into the POs that 2Checkout received payments for (representing the money that 2Checkout transfers to your account with your next payment). Access info on upcoming payments for POs already paid for by shoppers in the Accounting Summary area of the CP, under Reports center -> Accounting.
Payments for which the Download Invoice button is greyed out are associated with purchase orders. Downloadable invoices are available for all other payment methods used by shoppers.