15 C.F.R. § 200.109
(g) When a test number has been assigned prior to shipment to NIST, this number should be clearly marked on the shipping container. When a test number has not been assigned, an invoice, copy of the purchase order, or letter should be enclosed in the shipment to insure proper identification. The original purchase order should be forwarded as appropriate to:
Office of Measurement Services, National Institute of Standards & Technology, Washington, DC 20234; or to Measurement Services Clerk, National Institute of Standards & Technology, Boulder, CO 80303.