Consider tax liability due date on buy-now, pay-later promotions

We are a truck body manufacturer. As a promotion to sell certain bodies, we are offering customers an opportunity to buy a body with no money down for 60 days. On the 61st day, the entire sale price is due. On day one, title is transferred to the customer, but we receive no payment for the body until the 61st day. We know that we must pay tax on the sale of these bodies under IRC Section 4051, but we believe that we are not liable for the tax until the customer pays us. Are we correct?