Pennyweights (dwt) and grains (gr) are units of troy weight in the troy system of mass. Troy weight is not used any longer, except for the troy ounce, which is used for pricing gold, platinum and other precious metals. Postal rates were calculated in troy because money issued by individual states was unstable in value.
A grain is the smallest unit of troy weight, a pennyweight the second smallest. There are 8 grains in a pennyweight. One of the primary duties of a postmaster was to translate the value in local currency to its equivalent worth in troy using the most up-to-date exchange rates available.