How to get the Status of an Invoice

select APPS.Ap_Invoices_Pkg.GET_APPROVAL_STATUS(I.INVOICE_ID, I.INVOICE_AMOUNT,I.PAYMENT_STATUS_FLAG,I.INVOICE_TYPE_LOOKUP_CODE) FROM AP_INVOICES_ALL I where i.invoice_id='id of the invoice';

You have to set the Org-Id before you do that. This can be done with

call fnd_client_info.set_org_context('your org-id');