19 USC 340 - Consuls to exact proof of invoice

All consular officers are authorized to require, before certifying any invoice, satisfactory evidence, either by the oath of the person presenting such invoices or otherwise, that such invoices are correct and true. In the exercise of the discretion hereby given, the consular officers shall be governed by such general or special regulations or instructions as may from time to time be established or given by the Secretary of State.