If any amount of Illinois transfer tax imposed by this Act is not paid on or before the initial due date for the Illinois transfer tax return (without extensions), interest shall be charged and collected on the unpaid amount at the rate of 10% per annum from and after the initial due date and for the time thereafter as the tax is not paid, provided that the rate of interest on the outstanding amount of Illinois transfer tax permitted to be deferred or paid in installments under this Act shall be 6%.