Section 2. An excise is hereby imposed upon sales at retail in the commonwealth, by any vendor, of tangible personal property or of services performed in the commonwealth at the rate of 6.25 per cent of the gross receipts of the vendor from all such sales of such property or services, except as otherwise provided in this chapter. The excise shall be paid by the vendor to the commissioner at the time provided for filing the return required by section sixteen of chapter sixty-two C.