As we've covered here before, Tesla buyers are facing the expiration of the full federal plug-in vehicle tax credit that the cars are currently eligible for through Dec. 31.

Twice, Tesla has set deadlines for buyers to order their cars if they want to receive the full credit. Now Tesla is offering a carrot alongside the IRS's stick.

CHECK OUT: Tesla (again) says buyers must order today for full tax credit

For buyers looking for a Tesla, CEO Elon Musk says the company will offer a full refund if Tesla can't deliver on time. He said the company has a waiting list of customers hoping to get cars by the end of the year.

On Twitter, he said: "To be on the cancellation waitlist for delivery this year or if you want a display car, order at or visit our stores. Full refund if Tesla can’t deliver your car this year."

It seems likely that Tesla may have display cars available right up through the end of the year, but for buyers who want custom features, it may already be too late to be sure and get them before the tax credit is diminished.

Under the tax law, electric-car buyers are eligible for a $7,500 tax credit on electric cars from any automaker that has sold less than 200,000 of them. Once each automaker reaches 200,000 electric car sales in the U.S., the tax credit on its cars begins to wind down.

READ THIS: GM confirms it will reach EV tax credit ceiling this year—then phaseout begins

Tesla reached that ceiling in July, so the tax credit on its cars will be reduced to $3,750 starting January 1. On July 1, it will be reduced again to $1,875.

General Motors has reached the 200,000-car threshold this quarter and buyers of the Chevrolet Volt and Bolt EV will see their potential tax credits diminish to $3,750 in April. The Volt ends production in March and should be trickling out of dealer's lots by then. The Bolt EV will continue to be eligible for diminished credits until March 2020.