I just saw this in the AZT "E-News:"

"Once again, Saguaro National Park proves it is an innovator within the National Park system with the implementation of an eFee system that allows park visitors to pay for backcountry permits at the trailhead. This will prove especially beneficial for long-distance Arizona Trail thru-hikers.

“Visitors have expressed concern over the inability to pay fees at trailheads and find it cumbersome to travel all the way to a visitor center before starting a hike from the more remote trailheads,” said Park Superintendent Darla Sidles. “We are responding to that concern with a new mobile service to enable visitors to pay their fee from the trailheads utilizing their mobile device or from home before leaving for the park.”

Some of the park trailheads will include new signage with a QR code that will link people to a special webpage and explain how to pay fees using a mobile device as they enter the park. Alternatively, the park has set up a webpagewhere visitors can pay their fees before leaving home.

“Both of these eFee programs will make it easier for visitors to support the park through their entrance fees,” added Superintendent Sidles."

I remember Shenandoah and GSM NPs had an easy system for AT thru hikers, and the PCTA's permit system made it real easy out there, but getting proper hiking permits can be a problem on the lesser traveled trails. This is a real nice innovation for the AZT (at least for hikers who have smart phones, and near urban areas with phone service).