Your question is a difficult one facing AP organizations and the answer depends on your management's philosophy towards authorizations and how many invoices are processed without a purchase order (PO) reference. Many companies of your size require POs for purchases over a certain dollar amount. Amounts under this limit they often try to have purchased through a Pcard program that can result in a couple of good business practices.
- Numerous small dollar purchases are handled efficiently through the Pcard program by consolidating the small purchases and payments.
- Only authorized purchasers participate in the program and there are strict restrictions on any delegation of authority
- The Pcard program only allows the purchase of certain allowable items.
Beyond this solution you could solicit management's buy-in to make it mandatory that all invoices must have a legible signature with a title. If there is not an adequate signature, then the invoice gets bounced back. While this could delay the vendor payment, the department manager might get on board if they have a vendor calling them about payment or a manager asking why they cannot seem to add their legible title to an approval.
We also suggest you post your question to the Messaging/Forum section of the site to get feedback directly from your peers as to how they solved this problem successfully in their organizations.