Letters addressed to places abroad may be prepaid in this country either in money or stamps, but such payment must be made either wholly in stamps or wholly in money. The only exception to this rule is when the rate of postage includes a fractional part of a penny, for which, of course, there are no existing English stamps.
With certain exceptions, the only admitted evidence of the prepayment of a foreign letter is the mark agreed upon with the particular foreign country or colony.
When prepayment is _optional_, any outward letter (_e. g._ going abroad) posted with an insufficient number of stamps is charged with the deficient postage in addition, unless the letter has to go to Holland, or to the United States, or to a country through France, in which case it is treated as wholly unpaid, the postal conventions with these countries not allowing the recognition of partial prepayment. When, however, prepayment of the whole postage is _compulsory_, a letter, or aught else posted with an insufficient number of stamps, is sent (by the first post) to the Returned Letter Office.
Letters for Russia and Poland are also treated as wholly unpaid, if the full postage has not been paid in the first instance.
Letters to or from Ceylon, Australia, New Zealand, British West Indies (except Turk's Island), Honduras, and St. Helena, posted wholly unpaid, or paid less than one rate, are detained and returned to the writers for postage. If the letters should be paid with one rate (paid for half an ounce, for instance, when the letter weighs more than half an ounce), they are forwarded (except in the case of New Zealand), charged with the deficient postage and sixpence as a fine. Letters for New Zealand must be fully prepaid.
Letters for nearly all our remaining British colonies, if posted unpaid, either wholly or in part, are, on delivery, charged sixpence each in addition to the ordinary postage.
Letters intended to be sent by private ship should, in all cases, have the words "By private ship," or "By ship," distinctly written above the address. The postage of letters forwarded by private ship is sixpence--if the weight does not exceed half an ounce--and the postage must generally be prepaid. Exception is made to most of our North American and African colonies, to which places prepayment by private ship is not compulsory. (See table in the _British Postal Guide_.)