BOISE, Idaho — Seniors are snapping up so many lifetime passes good for U.S. national parks and other recreation areas ahead of a steep price increase later this month that some government agencies have run out and started issuing vouchers.

The America the Beautiful Lifetime Senior Pass has been available to buyers 62 and older for $10 for the past two decades. On Aug. 28, the price is going up 700 percent, to $80, after a measure passed by Congress late last year.

Recently retired Paul Dunham of Yuba City, California, picked one up last week at Craters of the Moon National Monument and Preserve in central Idaho.

“I said, ‘Hey, I’m a senior. Any decent rates?'” he said, expecting to pay $30 for a day visit to the high desert area known for its ancient lava flows.

Instead, for $10 he got the lifetime senior pass that’s recognized at more than 2,000 recreation areas run by six federal agencies that include the National Park Service, U.S. Forest Service and U.S. Bureau of Land Management.

“I don’t say many thanks to the U.S. government about anything,” Dunham, 64, said after discovering the deal. “But I will this time.”

Out-of-the-way Craters of the Moon still has the lifetime passes, but other areas that draw more visitors have run out, and officials authorized a rain check policy in mid-July. The policy allows people to use vouchers instead of the lifetime passes, and ultimately exchange them for one of the plastic, credit card-sized senior passes when they become available.

Some sites normally sell only a few hundred passes a year, Tom Crosson, the National Parks Service’s chief of public affairs, said in an email to The Associated Press. “Now, they are selling that many in a day.”

Agencies expect to sell up to 2 million of the passes by this fall, about two to three times what was sold last year, he said. Online and mail orders in 2016 were about 33,000 but have surpassed 250,000 so far this year and are expected to surge past 300,000 by the price-increase deadline.

Pass holders driving into areas with entrance fees can also bring in traveling companions for free. At some areas, the pass includes 50 percent discounts on camping, boat launching and other amenities.

Money raised by the price hike is intended for projects and programs aimed at enhancing the visitor experience at national parks and other areas.

The price has been $10 since 1994. The cost of a pass with similar benefits for those under 62 — but good for only one year — is remaining the same as last year, at $80.

John Freemuth, a Boise State University environmental policy professor and public lands expert, has one of the senior passes and said the bump to $80 isn’t much considering the adventures it makes possible.

“I still think it’s one of the best deals in America,” he said.

Craters of the Moon employee Rachel Amento said a local U.S Forest Service office ran out of the passes and has been sending buyers to Craters.

She said most of the park’s visitors are on their way to Yellowstone National Park and Grand Teton National Park. For motorists, the cheapest pass to those parks is good for seven days and costs $30. At Craters, the seven-day pass costs $15. Amento said adding it up plus other discounts still makes $80 a good deal for a lifetime senior pass.

“People will just have to adjust to the idea that they’ll have to visit more than one park to make it pay for itself,” she said.