"A single letter is one written on one sheet of paper sealed; a double letter is one sheet of paper which covers another sheet sealed up; a treble letter proportionately."—Calendar of State Papers (Domestic Series), 1658, p. 368.

Ordinance of 1657.
For every Letter—If Single.If Double.Per ounce.
s.d.s.d.s.d.
To or from any place within 80 miles of London020408
At a further distance than 80 miles030610
To or from Scotland040816
To or from Ireland061020
In Ireland—
  To or from any place within 10 miles of Dublin020408
  At a further distance than 40 miles040810

—H. Scobell, A Collection of Acts and Ordinances, London, 1658, p. 512.

Act of 1660 (12 Car. II, Cap. 35).
On Single
Letter.		On Double
Letter.		Per ounce.
From London—d.d.d.
  80 miles and under248
  Above 80 miles3612
To or from Berwick3618
From Berwick within Scotland—
  40 miles and under248
  Above 40 miles4812
To or from Dublin61224
From Dublin within Ireland—
  40 miles and under248
  Above 40 miles4812

N.B.—There were no cross posts. Between two towns not on the same post road, however near, letters could circulate only through London, and whenever a letter passed through London an additional rate was imposed, e.g. from Bristol to Exeter (less than 80 miles) a letter would be sent via London and charged two rates for over 80 miles.

1711 (9 Anne, Cap. 10).
Single.Double.Ounce.
From London—d.d.d.
  80 miles and under3612
  Above 80 miles4816
To Edinburgh61224
To Dublin61224
From Edinburgh within Scotland—
  50 miles and under248
  Above 50 miles and not exceeding 80 miles3612
  Above 80 miles4816
From Dublin within Ireland—
  40 miles and under248
  Above 40 miles4816