Final answer:
The control activity not usually performed in the accounts payable department is determining the mathematical accuracy of the vendor's invoice.
Step-by-step explanation:
The control activity that is not usually performed in the accounts payable department is 1) Determining the mathematical accuracy of the vendor's invoice.
Normally, determining the mathematical accuracy of the vendor's invoice is the responsibility of the accounts receivable department. This ensures that the correct amount is paid to the vendor.
The other control activities mentioned, such as having an authorized person approve the voucher, controlling the mailing of the check and remittance advice, and matching the receiving report with the purchase order, are all part of the standard practices in the accounts payable department for ensuring accurate and reliable payment processing.