You have to use the full path of the location of the procedure - it is probably not in the SAP<SID> schema. And also using quotes will help:
e.g.
call "_SYS_BIC"."system-local.private.TEST_DEMO_A4H/PAID_ON_DATE"(:iv_payment_date, :et_invoice_header, :et_invoice_item, :et_customer_info );
Cheers
Jens