Amazon CFO Tom Szkutak revealed that the company may be hiking the price of its popular Amazon Prime service—possibly by as much as $20 to $40 beyond the current $79 annual fee. Szkutak spoke during Amazon’s fourth quarter 2013 earnings call today.

The reason is obvious. Shipping is expensive, and Amazon’s current system is groaning under the burden of all the free shipping Prime allows. Szkutak said that transportation costs are up, and so are Prime shipments. (He said that Prime units have grown to 19 million from 1 million.) 

Prime membership also includes access to Amazon’s free video-streaming service and the ability to “borrow” some Kindle e-books for no additional cost.

In the same announcement, Amazon’s financial results fell short of Wall Street expectations, despite revenue growth of 20 percent to $25.59 billion.