Section 20. Articles furnished or services rendered under the preceding section shall be charged for, so far as practicable, at rates calculated to cover all costs and charges connected with the particular undertaking or service, and shall be paid for in cash at the time of delivery or performance. Any town acting under said section shall keep accounts, in the form prescribed by the director of accounts, covering separately, so far as practicable, the cost of the particular undertaking or service in respect to each commodity included therein; and a report in detail of receipts and expenditures connected with such undertaking or service shall be included in the annual report of the town treasurer.