付款, 應該以invoice付, 還是以statement付呢?

For better internal control, you should ask supplier the original signed invoices for payment arrangement.
You should also check with monthly statement sent from vendor for o/s invoices.
For any invoices which you are not yet received original doc., please better not pay for it. Since you not yet get proper approval for these invoices and appropriate verification by other department.

You can check the statement with all your purchases invoice on hand. Any difference, you can call your supplier for adjustment. After all, you can draw the cheque for payment at the month end.