What element is essential for a contractor's invoice to be considered proper?

A contractor's invoice is considered proper when it is certified. The certification serves as a declaration by the contractor affirming that the work performed and the costs incurred are accurate. This requirement helps ensure that the government only pays for services rendered and goods delivered, creating accountability in the contracting process.

When an invoice is certified, it typically includes a statement confirming that the costs are true and correct and that the contractor is authorized to submit it for payment. This certification may also imply that the contractor agrees to comply with all applicable laws, regulations, and contract provisions.

While elements such as timely submission and necessary signatures are important for processing efficiency and administrative purposes, they do not independently validate the invoice's correctness or the contractor’s liability, which is fundamentally guaranteed through the certification. Therefore, certification is the key requirement to establish the legitimacy and proper execution of the invoice.