Except as otherwise provided, the Governors are authorized to establish reasonable and equitable classes of mail and reasonable and equitable rates of postage and fees for postal services in accordance with the provisions of this chapter. Postal rates and fees shall be reasonable and equitable and sufficient to enable the Postal Service under honest, efficient, and economical management to maintain and continue the development of postal services of the kind and quality adapted to the needs of the United States. Postal rates and fees shall provide sufficient revenues so that the total estimated income and appropriations to the Postal Service will equal as nearly as practicable total estimated costs of the Postal Service. For purposes of this section, “total estimated costs” shall include (without limitation) operating expenses, depreciation on capital facilities and equipment, debt service (including interest, amortization of debt discount and expense, and provision for sinking funds or other retirements of obligations to the extent that such provision exceeds applicable depreciation charges), and a reasonable provision for contingencies.