Applicability of the New Rules. The new cargo securement rules apply to the same types of vehicles and cargo as the old rules, covering all cargo-carrying commercial motor vehicles (as defined in 49 CFR 390.5) operated in interstate commerce.
Most commercially-produced postcards measure approximately 4.21 inches by 5.47 inches. Less commonly, smaller postcards are typically 3.5 inches by 5.5 inches, close to the minimum size the USPS allows.
According to http://pe.usps.com/Archive/HTML/DMMA....htm#wp1002733 minimum thickness for a USPS postcard is 0.007 and maximum thickness is 0.016. Typical coated cover stock thickness is: 80 lb .008 100 lb .010 110 lb .011 120 lb .012 100 lb cover (270 gsm) is probably the most popular thickness for postcards.