The differing analyses of Post Office revenue result largely from their being based on consideration of the balance-sheet of the Post Office, as indicating whether postal charges are to be regarded as taxes.[746] The character of postal charges should not, however, be determined by reference solely to the amount of the surplus revenue. The true classification rests on the conception that the character of public revenue (including Post Office revenue) varies with varying circumstances.[747] The penny letter rate is a source of very considerable profit, and is therefore not a pure price. Nor can it be said that this penny rate, although it is the source of practically all the profit, is a pure tax. In the case of a large number of letters there is no surplus beyond the cost of the service, and often the cost is greater than the yield of the postage on the particular letters dealt with. In such cases the rate does not contain any element of tax. In other cases the proportion of surplus over cost which the rate yields is exceedingly large.[748] But in all cases it contains some element of remuneration for service rendered. That part of it which is appropriated to cover the cost of conducting the service is of the nature of a price for a service rendered. The remaining part (when found), after allowance has been made for the element of monopoly, is a tax.[749] But it does not exist in all cases. Three categories of letters are therefore found; and the letter rate in general may, according to the circumstances under which the service is rendered, be (1) of composite character, partly price and partly tax, (2) a pure price, (3) a mere fee. The other rates (excepting for the moment the parcel rate) have all for some specific purpose of State been fixed at a lower level than the letter rate; but, for the most part, without any nice adjustment to the cost of service. Consequently these subsidiary rates are not prices, and do not contain any element of taxation.[750] They are, however, charges made to individuals in respect of certain services performed by the State, and fall, therefore, under the heading of fees. The parcel rates in England and Germany may be put under the same heading. In both cases the service is conducted at a loss, and the charges cannot therefore be regarded as prices. In the United States and in Canada the law provides that the rates for parcels must in all cases be such as to yield a revenue sufficient to cover the cost of the service, and the presumption is therefore that in those countries the rates will partake of the nature of prices.[751]