When is Federal Excise Tax due on installment sales?

​​​​​​​We are a truck body distributor and are thinking about offering a special sales promotion to retail customers that offers the purchaser a chance to spread the cost of the body over a six-month period. In other words, the customer can pay the purchase price in six equal monthly installments. When buying a body under this promotion, the customer pays one-sixth of the purchase price, the title is transferred and the customer takes possession of the body. We know the body is taxable under Internal Revenue Code Section 4051, but we believe we would not be liable for the tax until the customer pays us the full purchase price. Are we correct?