Amazon on Friday announced that it has raised the price of its Prime membership program for those who subscribe on a month-to-month basis.

The plan previously cost $10.99 a month, but it will now cost $12.99 a month. That means the price of subscribing to the monthly Prime plan for a full year has jumped 18 percent, from $131.88 to $155.88. Those who currently subscribe to the monthly plan will see the price hike take effect on their first payment after February 18.

The e-commerce giant said it has also raised the rate of its cheaper Prime plan for students from $5.49 a month to $6.49 a month. The Prime Student plan launched this past October.

Amazon’s annual Prime plan will remain at its usual $99, keeping it a significantly better deal for those who can afford to pay the full fee upfront. The $49 annual rate for the Prime Student plan will stay the same, too, as will the $9 monthly fee for the Prime Video service that otherwise comes included in a full Prime plan.

In an emailed statement, an Amazon representative suggested the bump was due to the ongoing increase in items eligible for Prime’s expedited shipping perks, as well as the growing amount of content in Prime Video:

Prime provides an unparalleled combination of shipping, shopping and entertainment benefits, and we continue to invest in making Prime even more valuable for our members. The number of items eligible for unlimited Free Two-Day Shipping increased in recent years from 20 million to more than 100 million items. We have expanded Prime Free Same-Day and Prime Free One-Day delivery to more than 8,000 cities and towns. We also continue to introduce new, popular and award-winning Prime Originals, like The Grand Tour, Sneaky Pete, and the Golden Globe-winning The Marvelous Mrs. Maisel – all included with Prime Video. Members also enjoy a growing list of unique benefits like Prime Music, Prime Reading, exclusive products and much more. We will keep introducing new ways to make members’ lives even better.

Amazon introduced the monthly Prime plan in the US in early 2016 as an alternative to the relatively high cost of its annual fee, which it raised from $79 to $99 in early 2014.

Amazon has long been cagey about sharing exact Prime subscribers figures, but recent third-party estimates have pegged the service as having about 90 million members in the US alone. Exactly how many of those are month-to-month subscribers is unclear.

Amazon has stuffed Prime with various new features in recent years, and the membership program remains one of the company’s primary cash cows. Its members tend to spend far more on the site than non-Prime users.

As the company continues to dominate the online shopping market, though, it either feels that Prime’s previous monthly rate was unsustainable for the number of goods it provides—or that it’s big enough to raise the price of the monthly plan without much backlash from those who are already reliant on it. In any event, the company appears a bit more eager to have those interested in Prime lay down $99 upfront.